Authorities are investigating the communications of the New York truck suspect, hoping to learn about the circumstances that led to the attack and whether the alleged killer had assistance of any kind.

Police and FBI investigators have been trying to piecing together a complete picture of the path that led suspect Sayfullo Saipov to rent a truck and allegedly drive it through a crowded Manhattan bike path on Tuesday. They are looking for crucial clues to both explain the crime, and to ensure that others aren’t connected to the killer and capable of another attack. At the same time, a political battle has swelled over visa programs in the United States – such as the one Mr Saipov used to enter the country – and what to do with Mr Saipov should he be convicted, with President Donald Trump calling for the suspect to face the death penalty.

Federal prosecutors have filed charges against 29-year-old Mr Saipov including one count of providing material support to a foreign terrorist organisation and one count of violence and destruction of motor vehicle. In the charging document it said that Mr Saipov bagan to plan the attack a year ago and allegedly choose Halloween as he intended to kill as many people as possible and he had told investigators he “felt good” about the attack.

The federal court documents also alleged that the suspect had planned to target Brooklyn Bridge as well, and wanted to display Isis flags on the truck, but decided they would draw attention.

On Thursday, investigators indicated that they were talking with Mukhammadzoir Kadirov, who they believe may have pertinent information about the attack, sources told CNN. The FBI had said on Wednesday that that were seeking Mr Kadirov, before announcing less than 90 mins later that they had found him – they did not provide any further details.

Mr Kadirov released a statement to Associated Press through a person in touch with his family, calling the attack “sad and unbelievable”. He said “no human being who has a heart can do this.”

A law enforcement official said that Mr Kadirov was a friend of Mr Saipov and may not have any role in the case.

Those charges against Mr Saipov came after he allegedly turned a rented Home Depot truck on to a bike path in lower Manhattan on Tuesday before speeding up, leaving a gruesome trail of bodies and mangled bodies in its path. His attack ended after he smashed his truck into a school bus, and exited with a pellet gun and paintball gun.

Eight people were killed as a result of that attack, and at least 12 others were injured, some of them critically. Mr Saipov was shot by a police officer, and then taken to the hospital, where investigators say he has been cooperating.

New York Truck attack Show all 6 1 /6 New York Truck attack New York Truck attack Investigators inspect the truck following the attack Investigators inspect a truck following a shooting incident in New York on 31 October 2017. Several people were killed and numerous others injured in New York on Tuesday when a suspect plowed a vehicle into a bike and pedestrian path in Lower Manhattan, and struck another vehicle on Halloween, police said. A suspect exited the vehicle holding up fake guns, before being shot by police and taken into custody, officers said. The motive was not immediately apparent. Getty New York Truck attack A paramedic looks at a body along the bike path A paramedic looks at a body covered under a white sheet along the bike path 31 October 2017, in New York. A motorist drove onto a busy bicycle path near the World Trade Center memorial and struck several people Tuesday, police and witnesses said. AP Photo/Bebeto Matthews New York Truck attack The truck driver collided with a school bus Authorities respond near a damaged school bus Tuesday, 31 October 2017, in New York. A motorist drove onto a busy bicycle path near the World Trade Center memorial and struck several people, police and witnesses said. Two adults and two children were on the bus at the time. AP Photo/Bebeto Matthews New York Truck attack A young girl reacts to the scene A young girl reacts as police officers secure an area following a shooting incident in New York on 31 October 2017. Several people were killed and numerous others injured in New York on Tuesday after a vehicle plowed into a pedestrian and bike path in Lower Manhattan, police said. 'The vehicle struck multiple people on the path,' police tweeted. 'The vehicle continued south striking another vehicle. The suspect exited the vehicle displaying imitation firearms & was shot by NYPD.' DON EMMERT/AFP/Getty Images New York Truck attack Police officers arrive at the scene Police officers arrive at the scene following a shooting incident in New York on 31 October 2017. Multiple people were hurt in downtown Manhattan, US media reported after police confirmed that they were responding to reports of a shooting. Police said they had mobilized to the scene in Lower Manhattan and that one person was in custody, giving no further details. DON EMMERT/AFP/Getty Images New York Truck attack Emergency personnel respond to truck attack in lower Manhattan Emergency personal respond after reports of multiple people hit by a truck after it plowed through a bike path in lower Manhattan on 31 October 2017 in New York City. According to reports up to six people may have been killed. Kena Betancur/Getty Images

Investigators have since found that Mr Saipov came to the US legally in 2010 on diversity immigrant visa, which are awarded randomly to select countries to promote immigration from areas that don’t usually send immigrants to the US.

Since moving to America, Mr Saipov has worked as a truck driver and, more recently, as an Uber driver in Paterson, New Jersey, where he lives in a red brick apartment building near a mosque. Before moving to New Jersey, which appears to be a fairly recent decision, Mr Saipov had lived in Florida.

Investigators say that Mr Saipov, who allegedly yelled “allahu akhbar” after exiting the truck, had seemingly radicalised himself online while living in the US. Mr Saipov had numerous Isis propaganda videos on his mobile phone, including footage of terrorists beheading people. Officials said that Mr Saipov appears to have followed directions for the attack that Isis published last year almost exactly.

According to court filings, Mr Saipov later told investigators that he decided two months ago to use a truck, and had rented the truck previously in order to get used to the vehicle’s handling.

Mr Trump waded back into the issue on Thursday reiterating the view that Mr Saipov should face the death penalty for the attacks, raising the rare prospect of a death penalty trial in New York, which bans that punishment at state level. Mr Trump’s national security adviser, General HR McMaster, responded to those statements saying that the administration is looking to provide Mr Trump with prosecutorial options.

Mr Trump and his administration have backed away from the prospect of jailing Mr Saipov in the Guantanamo Bay prison, which the President said previously that he would consider. Sending Mr Saipov there would be an unprecedented step, as nobody arrested in the US has been sent to the military prison.

Also on Thursday, Attorney General Jeff Sessions told members of law enforcement in New York, in a visit scheduled before the attack that the US justice system can handle suspects like Mr Saipov.

He noted over 500 defendants have been convicted of terrorism-related crimes since the Sept. 11, 2001, attacks.

Mr Saipov is being held without bail at a Manhattan federal lockup next to the courthouse. His attorney, David Patton, has said he hopes “everyone lets the judicial process play out.”

On Wednesday, Mr Trump called the US justice system a slow-moving “joke” and “laughingstock” and said he would be open to seeing Saipov transferred to the U.S. prison at Guantanamo Bay, Cuba, where enemy combatants are tried by military tribunals.

But the president reversed course a day later, tweeting that “statistically that process takes much longer than going through the Federal system.”

The President has also questioned the wisdom of having a visa program that let Mr Saipov enter the country nearly seven years ago.

“The people put in that lottery are not the country’s finest,” Mr Trump said on Thursday, adding that the visa program creates “significant vulnerabilities” for American national security.

Mr Trump also called on Congress to immediately “terminate” the program