The U.S. Federal Bureau of Investigation(FBI) said the shooting of former U.S. President Donald Trump during Saturday's rally in Pennsylvania is believed to have been solely carried out by suspected gunman, Thomas Matthew Crooks.

According to The Associated Press and other outlets on Sunday, FBI officials investigating the shooting said they have yet to find evidence of the 20-year-old deceased suspect having suffered a mental illness or posing a threat online. He had never been under investigation by the FBI.

While handling the incident as an assassination attempt, the FBI said they remain open to the possibility of the shooting being an act of domestic terrorism, although the suspect's connection to any particular ideology has yet to be verified.

The investigators said the AR-15 rifle used in the attack was purchased legally, while the bomb-making materials found inside the suspect's vehicle were sent to an FBI lab for further analysis.

Meanwhile, AP also reported that just before firing shots at Trump, Crooks had pointed his rifle at a local law enforcement officer who had spotted him at a rooftop near the rally site.

As the officer retreated down the ladder, Crooks reportedly began firing at the former president, which prompted U.S. Secret Service gunmen to shoot and kill him.