New York bomb suspect acted alone: FBI

The suspect in the bombing this month in New York's Chelsea neighbourhood appears to have acted on his own, with no connection to a militant movement, the FBI said on Tuesday.

"We see so far no indication of a larger cell or the threat of related attacks," FBI director James Comey testified at a Senate committee hearing.

The suspect in the 17 September bombing that left 31 people wounded, and Ahmad Khan Rahami was arrested two days after the attack.

US prosecutors, in a 13-page indictment on 20 September, slapped him with four charges, including use of weapons of mass destruction.

In addition to the New York attack, he is charged with a pipe bombing, also on 17 September, in Seaside Park, New Jersey, and planting several other bombs.

A naturalised US citizen born in Afghanistan, Rahami made several trips in recent years to Afghanistan and Pakistan.

The terror charges came after the Federal Bureau of Investigation admitted it had investigated Rahami for terrorism in 2014 following a complaint from his father, but found no link to radicalisation.

Comey said the FBI is seeing a slight slowdown in new US terror investigation cases, but some 1,000 probes are currently ongoing.

"I hope that it's going to... head downward but it has not headed downward yet," he told the Senate Homeland Security and Governmental Affairs Committee.

By contrast, he said, the number of people leaving the country to join the Islamic State group in Syria or Iraq has fallen sharply.

"Where we used to see eight or 10 people from the United States trying to go to the so-called caliphate, we're now down to one or none a month," he said.