President Donald Trump has said that finding the “sick” people who have sent bombs to various locations in south central Texas is a top priority, seeking to reassure residents there is a heavy federal and local police presence to put an end to the series of attacks in Austin and now San Antonio.

“We have a lot of power down there, it’s not easy to find,” Mr Trump said from the Oval Office.

“We have to produce, we have to find these very sick people.”

The comments came as officials continue a desperate search for clues and answers after a sixth explosion was reported in Texas. Officials said soon after that that explosion "does not appear to be related" to the package bombings.

Earlier on Tuesday, a package exploded within a FedEx depot outside of San Antonio on Tuesday, spreading more fear after two lives have already been claimed and four others injured in previous attacks. Nobody was hurt in the explosion at the FedEx depot.

Rep Michael McCaul, a Texas Republican who chair the House Homeland Security Committee, told the Associated Press that he had learned investigators had obtained surveillance videos that “could possibly” show the suspect.

Austin bombings: Police confirm use of tripwire

Federal officials have swarmed to Austin, where packages have been detonated this month in a series of attacks that police say are likely related and have become increasingly sophisticated. The package that exploded on Tuesday outside of San Antonio was en route to Austin, and had originated in Austin as well, Texas attorney general Ken Paxton told local media. Federal investigators later said they believe that the explosion was likely related to the Austin attacks.

But officials have provided little indication at times that they are any closer to apprehending a suspect, and have repeatedly asked the public to come forward with any information that could end this string of bombings.

Fourth Austin bombing: in pictures Show all 8 1 /8 Fourth Austin bombing: in pictures Fourth Austin bombing: in pictures Investigators work at the scene of a bombing on Dawn Song Drive in the Travis Country neighborhood. Rex Fourth Austin bombing: in pictures Police tape marks off the neighborhood where the package bomb went off. It is the fourth similar bombing in three weeks in the Austin area. Getty Fourth Austin bombing: in pictures A police crime scene van arrives near the site of the explosion. Police warned nearby residents to remain indoors overnight as investigators looked for possible links to other deadly package bombings elsewhere in the city this month. AP Fourth Austin bombing: in pictures FBI and police investigate a bombing at the intersection of Republic of Texas and Mission Oaks boulevards. Rex Fourth Austin bombing: in pictures Officials work on the site of the explosion. AP Fourth Austin bombing: in pictures Members of Austin Police Department block off roads, Reuters Fourth Austin bombing: in pictures Police lines are seen blocking off part of Republic of Texas Boulevard. Reuters Fourth Austin bombing: in pictures Police maintain a cordon near the site of the explosion in southwest Austin. Reuters

“We are clearly dealing with what we expect to be a serial bomber,” Brian Manley, Austin’s interim police chief, said on Monday during a press conference after the fourth explosion on Sunday night, which was detonated by a trip wire. “The belief that we are dealing with someone who is using trip wires shows a higher level of sophistication, a higher level of skill.”

Police have repeatedly warned residents to stay away from any suspicious package, and said that even going close to one could be dangerous if it uses a trip wire – which could be any sort of thin thread and difficult to see. Austin Police Department has received 1,257 calls from concerned residents who have received potentially suspect packages in the past eight days, according to a tweet from the department.

A $115,000 (£82,000) reward has been offered for information leading to the arrest or conviction of the people responsible for the explosions in Texas, which has led the FBI to send at least 300 agents to Austin, in what FBI special agent Christopher Combs said was an “unprecedented” threat response in the city.

Austin mayor Steve Adler said he thinks the Sunday explosion is likely to increase tension in the city, and that concern is justifiable even though he says residents should feel more secure because of the heavy response from police and federal agencies.

“That concern is legitimate and real,” Mr Adler said on Monday. “That anxiousness is going to continue until we can find the answer.”

As the city of not even one million grapples with the several unknowns surrounding the package bombs, residents are understandably “scared” and “worried” over the seemingly random nature of the separate events, one resident told The Independent.

Neha Shah, who lives in Austin, said she has not seen any extra cautionary security changes at her office but her apartment building sent a notice that read: “The leasing office will still accept packages from known delivery services but will not accept any random deliveries from unknown parties. At this time, we will not be delivering packages to apartments”.

The notice added to the tense atmosphere and she had seen “extra patrolling” of police around the city.

Ms Shah has been attending the annual South by Southwest (SXSW) festival in town as well. For the most part, security at the festival had not changed in light of the three Austin package bombs and one San Antonio explosion. However, a bomb threat had been emailed to the Fair Market event venue, which was supposed to host a SXSW concert by the hip hop group The Roots.

Police arrested the man suspected of sending the threat but it remains unclear whether it or he are connected to the other bombs. The concert was cancelled due to security reasons. Two of the victims were African-Americans as are members of the band, but police have not said the bombings are a hate crime or race-related.

The first explosion occurred on 2 March, killing 39-year-old Anthony Stephan House after he brought a package in from his front porch. Mr House was taken to a nearby hospital but was later pronounced dead. At the time, police determined that the incident was an isolated event but that the FBI and agents from the Bureau of Alcohol, Tobacco, Firearms and Explosives were assisting in an investigation.

Two more bombs hit the city on 12 March, which led Austin police to say that those three incidents were likely related. The first on that day killed 17-year-old Draylen Mason and injured his mother, after Mr Mason brought the package in from the front step of his home to his kitchen, where it detonated. The second bomb, hours later, injured a 75-year-old woman in a different part of the city.