The father of the Afghan-born man arrested after weekend bombings in New York and New Jersey reported concerns about his son to the FBI in 2014, but it found no "ties to terrorism" and dropped its inquiry, the father and the agency said on Tuesday.
US authorities were investigating whether Ahmad Khan Rahami, the naturalised American citizen who was arrested on Monday in New Jersey after a shootout with police, had accomplices in the bombings or if he picked up militant Islamic views during trips to Afghanistan and Pakistan.
"The investigation is active and ongoing, and it is being investigated as an act of terror," US Attorney General Loretta Lynch said in Lexington, Kentucky.
FBI officers work at the site where Ahmad Khan Rahami, inset, was arrested after a shoot-out. Photo: AFP
Mr Rahami, 28, was suspected in the weekend bombings, including a blast on Saturday night in New York City's Chelsea neighbourhood that wounded 29 people, and another on the New Jersey shore that injured no one earlier that day.
His father, Mohammad Rahami, briefly emerged on Tuesday (local time) from the family's restaurant in Elizabeth, New Jersey, about 30 km west of New York City, and told reporters, "I called the FBI two years ago."
The Federal Bureau of Investigation said in a statement that it began an assessment of the younger Rahami in 2014 based on comments his father made about his son after "a domestic dispute."
"The FBI conducted internal database reviews, interagency checks, and multiple interviews, none of which revealed ties to terrorism," the FBI said.
One US law enforcement official said the elder Rahami met twice with the FBI, first saying he was worried his son was hanging out with people who might have connections to militants, but two weeks later contending his real concern was that the son was associating with criminals.
Another law enforcement official said the father "recanted the whole story" about his son associating with terrorists.
The comments by the officials, who spoke on the condition of anonymity, showed Mr Rahami had been brought to the attention of the FBI before the bombings much like others who have carried out attacks in the United States in recent years.
The FBI's Miami office had investigated Omar Mateen, the man who killed 49 people at a gay nightclub in Orlando in June and expressed allegiance to the leader of the Islamic State organization, for 10 months and interviewed him twice but found no evidence of a crime or connection with a militant group.
The FBI was also alerted to 2013 Boston Marathon bomber Tamerlan Tsarnaev, including a warning from Russian's government.
Mr Rahami was arrested on Monday in Linden, New Jersey, not far from Elizabeth, where his family lived above their storefront First American Fried Chicken restaurant.
Mr Rahami and two police officers were wounded in the exchange of gunfire before his arrest. Mr Rahami was listed in critical but stable condition, and police had not yet been able to interview him in depth, New York Police Department Commissioner James O'Neill said.
Notebook musings
Two US officials said Mr Rahami had a small notebook on him when he was apprehended, which one of the officials said contained "ideological" musings.
The notebook, punctured by a bullet hole and stained with blood, contained references to killing nonbelievers and mentioned American-born Muslim cleric Anwar al-Awlaki, a leading al Qaeda propagandist who was killed in a US drone strike in Yemen in 2011, the New York Times reported, citing an unnamed law enforcement official.
Two US officials told Reuters Mr Rahami had travelled to Afghanistan and to Quetta, Pakistan, a city where support for the Taliban is significant.
The official, and other US security sources, said Mr Rahami underwent additional security screening upon returning from abroad but passed each time. One of the officials, who specialises in counter-terrorism, said the "secondary" screening included asking Mr Rahami where he had gone and for what purpose.
Mr Rahami's wife left the US a few days before the bombings, CNN reported, citing a law enforcement source.
Prosecutors in Union County, New Jersey have charged Mr Rahami with five counts of attempted first-degree murder and two second-degree weapons counts. More charges were expected to be brought against Mr Rahami in federal court.