The FBI has warned there could be more pipe bomb devices on their way to high-profile Trump critics as a sting of Hollywood A-listers have been put on alert.

FBI Assistant Director William Sweeney warned the 'American public to remain vigilant as it does remain possible further packages have been or could be mailed.'

'These devices should be considered dangerous,' he added.

It comes as it's revealed the crude bombs posted this week were sent through a major United States Postal Service mailing facility in South Florida.

All ten bombs sent to the Clintons, former President Obama, CNN and other prominent Democrats are believed to have come through the Opa-Locka processing and distribution center.

New York City Mayor Bill de Blasio said the city had already stepped up its police presence 'to show very vividly that New York City takes these issues seriously'.

The crude bombs lacked a 'triggering mechanism' suggesting they were never meant to explode, experts who have seen photos of the devices claim

An NYPD bomb squad vehicle leaves an area outside Time Warner Center on Wednesday where a suspicious package led to the evacuation of CNN's offices

It's been revealed the bombs were sent through a major mail facility in Opa-locka, Florida, pictured

Police are pictured outside the U.S. Post Office Royal Palm Processing & Distribution Center Thursday

It was revealed last night that the device delivered to CNN's New York office was delivered by a courier, a law enforcement official told CNN.

Billionaire investor George Soros, who has backed Democrats and was the first to be targed by what has been dubbed the MAGAbomber, also uses a courier service that picks up his mail at a post office before it is delivered to his home.

On Monday the first pipe bomb was found in his mailbox at his Manhattan home.

Since that discovery a further nine suspicious packages have been found at addresses linked to prominent Democrat politicians and supporters including Barack Obama, Joe Biden, the Clinton family, actor Robert De Niro and the CNN news network.

Other Hollywood A-listers such as Arnold Schwarzenegger, Meryl Streep, Alec Baldwin, Cher and Ben Stiller, who have also spoken out about Trump's policies are said to have increased security over fears they could be targeted next.

Figures such as Amazon owner Jeff Bezos and British Last Week Tonight on HBO host, John Oliver, are also on the list of potential targets who have been put on alert.

When asked at a press conference last night whether there might be more bombs that have yet to be discovered, New York police commissioner, James O'Neill, said it was not known if more high-profile President Trump critics will be targeted.

He said: 'With the postal services that are here, we're doing our best to make sure that if there are any out there we identify them quickly.'

FBI and canine units were deployed to the facility Thursday as the search continues for the person or persons behind the bombs

The bombs seized on Wednesday, each with a small battery, were about six inches long and packed with powder and broken glass, according to x-ray images (pictured above)

New York City Police vehicles parked outside the office of the The New York Times yesterday where security was ramped up after explosive devices were sent to top Democratic politicians and the CNN headquarters

Police investigate a suspicious package inside the Time Warner Center at Columbus Circle in New York

The FBI called the packages 'potentially destructive devices' with what is believed to be pyrotechnic powder found.

A law enforcement official familiar with the investigation told CNN a suspicious powder found in one parcel was tested and initially deemed not to be anthrax or another known toxic agent, but that the substance would undergo more testing.

The FBI said on Wednesday the original packages were mailed in manila envelopes and covered in bubble wrap inside.

A statement from the agency said: 'They were affixed with computer-printed address labels and six Forever stamps. All packages had a return address of "DEBBIE WASSERMAN SHULTZ" (sic) in Florida.'

Wasserman Schultz, a Florida congresswoman, is the former chairwoman of the Democratic National Committee and often criticizes the president.

The packages were taken to the FBI lab in Quantico, Virginia, for examination.

FBI and canine units were deployed to the Opa-Locka mail facility yesterday as the search continues for the suspects behind the bombs, which is the main mailing facility in Miami-Dade County.

Opa-Locka once had the highest rate of crime for any city in the US in 2004 and continued to deteriorate.

While in the state of Florida there is a one in 232 chance of being a victim of a violent crime, in Opa-locka those chances increase to one in 44.

A ranking from May showed the city in the top five of 'most dangerous' cities in the US, while East St Louis came in first.

'We are investigating all this with great precision. I can say with certainty that we will identify and arrest the person or people responsible for these acts,' New York police Commissioner James O'Neill said at a news conference Thursday. 'Law enforcement professionals do not care about the politics involved in these acts.'

Another law enforcement official told CBC News: 'All arrows pointing to Florida.'

Shortly before clearing the scene, New York City police officers block the escalator to the second floor as shoppers at the Time Warner Center

Investigators swarmed two Delaware mail facilities on Thursday morning after intercepting packages addressed to former Vice President Joe Biden

The building on Greenwich Street in Tribeca was evacuated after the suspicious package was found early Thursday. The building contains Robert De Niro's Tribeca Grill restaurant and his film production company

Experts who have seen photos of the devices claim the crude bombs lacked a 'triggering mechanism' suggesting they were never meant to explode.

There are no confirmed details yet on why none of the packages sent to the likes of the Clintons and Obamas exploded but the FBI had previously described them as 'potentially destructive devices'.

Authorities described the devices as crude while security experts said their goal may have been to create fear rather than to kill.

'What they wanted to do was scare people, cause disruption, but not necessarily hurt anybody, because if they'd wanted to hurt people the bombs would have gone off,' Matthew Bradley, a former CIA agent, told Reuters.

TIMELINE: Packages target prominent Democrats and Trump's foes Monday: George Soros Pipe bomb is found at George Soros' home in Katonah, New York on Monday. It was located in a mailbox outside the gates of the billionaire's compound. Tuesday : Hillary Clinton Suspicious package is discovered at Hillary and Bill Clinton's home in Chappaqua, New York, Tuesday night. The device addressed to Hillary was found during a routine mail screening. Bill was at the home but Hillary was campaigning for Democrats in Florida. Wednesday: Barack Obama Secret Service intercept a package en route to Obama's Washington D.C. residence early Wednesday. It was also discovered during routine mail screenings. Wednesday: CNN/John Brennan Suspicious package addressed to former CIA director John Brennan is found in the mail room of CNN's New York Bureau on Wednesday. CNN evacuated after a pipe bomb and an envelope of white powder was discovered. Brennan has worked as a television analyst. Wednesday: Eric Holder/Debbie Wasserman Schultz Bomb squad is called to Florida office of Congresswoman Debbie Wasserman Schultz on Wednesday after package is found. Packages that were intercepted all had Schultz's office listed as a return address. Package at her office was sent to former Attorney General Eric Holder but was mailed to the wrong address, so was returned to her instead. Wednesday: Maxine Waters Suspicious package addressed to California congresswoman Maxine Waters was intercepted Wednesday afternoon by Capitol Police. Another package was sent to Waters in California. Thursday : Joe Biden Authorities locate two suspicious packages addressed to the former VP at mail facilities in Delaware on Thursday. Thursday: Robert De Niro New York Police swarm a building associated with the Hollywood actor in Tribeca early Thursday after a suspicious package was found. Advertisement

Investigators are trying to determine if the devices were intended to detonate or simply to sow fear. Officials told the AP that the devices were not rigged like a booby-trapped package bomb that would explode upon opening. They had timers and batteries but never went off.

The officials were still uncertain whether the devices were poorly designed or never intended to cause physical harm.

Experts who have seen photos of the devices say there isn't a triggering mechanism, but the presence of suspected pyrotechnic powder is one of the reasons they are considered 'potentially destructive devices', CNN reports.

Explosives expert and University of Rhode Island chemistry professor Jimmie Oxley said: 'My personal feeling is that this is some kind of a wacko, rather than an organization trying to accomplish something. Evil, mad, has a grudge. A little mix of all of these.'

The bombs seized on Wednesday, each with a small battery, were about six inches long and packed with powder and broken glass, according to a law enforcement official who viewed X-ray images.

The devices were made from PVC pipe and covered with black tape.

Initial analysis showed that white powder sent with some of the packages to New York was not a biological threat, according to FBI assistant director William Sweeney.

Authorities are currently investigating whether the series of pipe bombs sent to prominent Democrats and Trump critics were mailed from Florida.

A federal law enforcement source said investigators were focusing on leads the packages were sent from Florida as the FBI tries to determine where parts for the pipe bombs were purchased from.

All the suspicious packages were believed to have gone through the U.S. Postal System at some point.

The nationwide investigation widened on Thursday after three new packages addressed to Robert De Niro and Joe Biden were seized in New York and Delaware.

The latest devices are similar to others found this week targeting prominent Democrats and some of Trump's biggest foes, including the Obamas, Clintons and CNN.

Authorities were scrambling to find those responsible for the 10 devices that politicians on both sides of the aisle have branded terrorism as they warned that more packages could be out there.

Former President Jimmy Carter has been told by law enforcement to remain on alert because he could be a target, ABC reports. Authorities have also been contacting other former officials warning them to be vigilant.

The FBI is sharing very few details publicly about the investigation and have said nothing about possible motives or suspects.

Ten packages have so far been seized in New York, Washington D.C., Florida and California.

The pipe bomb sent to CNN featured an image of a parody ISIS flag popular on right-wing websites. Instead of featuring Arabic characters, the image contained the silhouette of three women, and comedian Larry the Cable Guy's catchphrase 'Get 'Er Done'.

Investigators examining the explosive devices will be working to glean forensic clues to help identify who sent them, gathering fingerprints and DNA evidence while tracking the origin of the packages and the components used to make the bombs.

New York City police investigate a suspicious package inside the Time Warner Center where a suspicious device was found

A member of the Miami-Dade County Bomb Squad is shown outside a postal facility Thursday

It's reported that all 10 of the mail bombs recently sent to former President Obama and others may have been sent through the facility

Among the first steps for investigators will be retracing the path of the packages through the postal system or courier service used to deliver them.

The U.S. Postal Service operates a sophisticated imaging system that photographs the outside of each piece of mail processed across the country and can be used to determine the specific location of where it was sent.

New York Governor Andrew Cuomo told CNN some of the packages in his state were hand-delivered and that authorities were scrutinizing security camera videos for signs of the bomber.

The packages discovered Thursday set off a new wave of alarm.

A law enforcement official said a person working at Robert De Niro's Tribeca office called police after seeing images of a package bomb sent to CNN and recalled seeing a similar package addressed to the actor.

De Niro's office is located in a building that also includes his upscale Tribeca Grill restaurant and his film production company.

The packages addressed to Biden were intercepted just hours later at Delaware mail facilities in New Castle and Wilmington.

In a similar pattern, both Biden and De Niro have been sharply critical of Trump.

The actor dropped an expletive insult at Trump at this year's Tony Awards and also apologized to Canadians for the 'idiotic behavior of my president.' Biden said last week that Trump may not 'know what he's doing' and coddles dictators.

All the people who have been targeted by the serial bomber are frequently maligned by right-wing critics, including Barack Obama, Hillary Clinton and Obama's Attorney General Eric Holder.

Former CIA Director John Brennan, prominent billionaire Democratic Party donor George Soros and California Representative Maxine Waters, an outspoken critic of Trump, were also targets.

The first crude bomb to be discovered was delivered Monday to the suburban New York compound of George Soros. The liberal billionaire and major contributor to Democratic causes has previously called Trump's presidency 'dangerous'.

The US Secret Service intercepted packages addressed to Obama at his Washington home and to Hillary Clinton at her home in Chappaqua, New York where she lives with husband Bill.

A member of the bomb squad investigates a suspicious package at a postal facility in New Castle, Delaware that was addressed to former Vice President Joe Biden

The package to Clinton was found late on Tuesday and one addressed to Obama was recovered early on Wednesday. The Secret Service said both were intercepted during routine off-site mail screenings. They said Obama and Clinton were not at risk.

The CNN bureau in New York received a package addressed to John Brennan, who has appeared as a CNN analyst, leading police to evacuate the Time Warner building in a busy Manhattan neighborhood near Central Park.

Additional packages were sent to frequent Trump critics, California congresswoman Maxine Waters and former Attorney General Eric Holder.

One package addressed to Waters was intercepted in Washington D.C., while the other was found at a mailing facility in Los Angeles where her office is located.

The parcel intended for Holder ended up rerouted to the return address printed on all the packages - Schultz's office in Sunrise, Florida.

As authorities continued the manhunt for those behind the attacks, experts speculated on the mastermind behind the pipe bombs.

Oxley, the explosives expert from Rhode Island University, said she suspected a single bomber, whereas former CIA agent Bradley said multiple people were probably involved because 'it's hard to coordinate that mail delivery on your own'.

Bradley ruled out a dedicated, smart and efficient killer such as Ted Kaczynski, the notorious Unabomber who killed three and injured 23 in a three-decade campaign using sophisticated devices sent through the mail.

'There's a little bit of disconnect, mentally for the person who's doing this, it doesn't sound like it's a cold calculating person like Ted Kaczynski,' Bradley said.

'This is more amateur than professional, but there's also professionals who could have wanted it to look that way.'

In the most recent similar incident, five relatively advanced package bombs exploded in Austin, Texas, back in March. Most were sent through Federal Express and triggered by movement. The suspected serial bomber Mark Conditt, 23, later blew himself up.

Police stand in front of the Time Warner building where an explosive device was found at the CNN's offices in New York on Wednesday

The package sent to Hillary and Bill Clinton's property in Chappaqua, New York was found by an employee who screens their mail. Police guarded the property early Wednesday

Experts said that in contrast to the Unabomber and Conditt, this week's would-be bomber used packages that were poorly designed and suspicious in appearance. The bombs seized this week, some of which were intercepted at U.S. Postal Service locations, were inside envelopes and were 6 to 8 inches long.

'The small size restricts what materials might actually function in it,' said Oxley, who has built 130 pipe bombs at the request of the FBI and the Bureau of Alcohol, Tobacco, Firearms and Explosives to aid investigators.

The reported presence of polyvinyl chloride (PVC) in at least one of the devices led her to believe the bomb builder wrongly thought the relatively lightweight material would not be detected by X-ray.

'I would prefer to think this was a scare. But this has to be taken seriously,' she said.

Larry Johnson, a former head of criminal investigations for the U.S. Secret Service who also served as a special agent in charge of the presidential protective detail, told the AP that bomb makers usually leave evidence behind.

Even the most careful bomber is likely to leave behind genetic material that could be used to identify them, especially traces of sweat, saliva or skin cells. There may also be fingerprints or hair.

A device addressed to former President Obama's home in Washington D.C. was intercepted by Secret Service. Law enforcement were spotted outside the home early Wednesday morning

The bomb packages were sent as the nation prepared for the November 6 elections that will decide whether Democrats take control of one or both houses of Congress from Republicans and deny Trump the majority his party now holds in both chambers.

Some top Democrats have called the threats a symptom of a coarsening brand of political rhetoric promoted by Trump, who also condemned the acts.

As the scope of the attack became clearer on Wednesday, Trump decried political violence during a scripted event in the White House East Room as other members of the administration said it has no place in American society.

'Let's get along,' Trump said at a Wisconsin rally on Wednesday night. 'By the way, do you see how nice I'm behaving tonight? Have you ever seen this?'

But in the same breath, he blamed the media and kept up the argument on Twitter on Thursday that the media plays a role.

'A very big part of the Anger we see today in our society is caused by the purposely false and inaccurate reporting of the Mainstream Media that I refer to as Fake News,' Trump wrote, without directly referencing the explosive devices.

'It has gotten so bad and hateful that it is beyond description. Mainstream Media must clean up its act, FAST!'

Trump's tweet attacking the media comes just one day after he called for unity after the first devices were found.

The U.S. Post Office Royal Palm Processing & Distribution Center is the largest mailing facility in Miami-Dade County

New York Police blocked off streets in Tribeca early Thursday to haul away a suspected explosive device in a containment vessel (above) that had been sent to actor Robert De Niro

Authorities removed a second suspicious package addressed to Biden after it was found at a mail facility in nearby Wilmington, Delaware

'I just want to tell you that in these times, we have to unify... we have to come together and send one very clear, strong, unmistakable message that acts or threats of political violence of any kind have no place in the Untied States of America,' he said.

The hashtag #MAGAbomber became a top trending Twitter hashtag on Wednesday.

White House press secretary Sarah Huckabee Sanders called it 'disgraceful' to suggest the president bears any responsibility for the packages sent to his opponents. She told reporters on Thursday that there was a big difference between 'comments made and actions taken'.

Several politicians, including Senate Majority Leader Mitch McConnell and New York Mayor Bill de Blasio, described the serial bombs as an act of terrorism.

'Someone is trying to intimidate. Someone is trying to quash voices in this country using violence,' De Blasio said.

'I am confident that we will find the perpetrator or perpetrators.'

Some leading Democrats were quick to accuse Trump himself of stoking the potential for political violence by frequently engaging in hyper-partisan, vitriolic rhetoric.

Democratic party leaders in Congress said Trump's call for unity rang hollow because of his past statements condoning acts of violence.

Last week, Trump, who has joined other Republicans in accusing Democrats of encouraging 'mob' tactics, heaped praise on a Montana congressman who assaulted a reporter in 2017.

At an event in Texas on Wednesday, Brennan faulted Trump for frequent inflammatory rhetoric.