"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 September 17 bombing that left 31 people wounded, Ahmad Khan Rahami, was arrested two days after the attack.
US prosecutors, in a 13-page indictment on September 20, 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 September 17, in Seaside Park, New Jersey, and planting several other bombs.
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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 radicalization or extremist sympathies.
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.