The March 22 attack on a concert hall in a Moscow suburb killed at least 144 people, the deadliest in Russia in 20 years. A branch of Islamic State claimed responsibility, but Russian President Vladimir Putin, without citing evidence, has sought to blame Ukraine.

US officials have been worried about the possibility of an attack carried out by an individual or small group inspired by the war between Israel and Hamas in Gaza.

But the FBI is growing concerned about a more coordinated attack following the concert massacre in Russia, Wray said in his testimony before a subcommittee of the House Appropriations Committee.

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At the end of the 2023 financial year, the FBI had 4,000 international terrorism investigations open, according to written testimony by Wray.

Of increasing concern “is the potential for a coordinated attack here in the homeland, akin to the Isis-K attack we saw at the Russia Concert Hall just a couple weeks ago,” Wray told lawmakers in a shorter version of his written testimony.

Wray cited the law enforcement agency’s concerns about terrorism to help persuade lawmakers to boost funding for the FBI, though he is likely to face strong resistance from congressional Republicans.

The FBI has become a prime target for Republican former president Donald Trump and his allies, with Trump alleging the bureau has unfairly targeted him while going soft on his political enemies.

He has called on Congress to slash the agency’s funding, and he has referred to the Justice Department and its FBI component as “vicious monsters”.

The rising political rhetoric against the FBI has led to an increase in threats against the bureau and its employees. Earlier this month, for instance, a South Carolina man was arrested after he tried to ram his car into its Atlanta office.

“We have seen a substantial jump in threats towards FBI personnel and facilities from financial year 2022 to financial year 2023,” Wray told lawmakers. “In fact, we created a dedicated unit to try to deal with those issues.”

Wray, who was appointed by Trump in 2017, asked lawmakers to help get the bureau’s budget “back on track” after its fiscal 2024 budget fell US$500 million short of what was needed to sustain its efforts.

Wray also pressed lawmakers to renew a US surveillance programme set to expire this month, calling it an indispensable tool against US adversaries. A modest overhaul of that programme was blocked in the House on Wednesday amid concerns from members of both parties that it did not go far enough in curbing the government’s surveillance powers.

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“It’s critical in securing our nation, and we are in crunch time,” Wray told lawmakers.

Trump and his allies have called for the surveillance programme to be shut down, after a different provision of the law – known as the Foreign Intelligence Surveillance Act, or FISA – was used to intercept communications with one of his 2016 campaign advisers.

“I’ll be honest with you, and this pains me to say this, but I don’t trust you,” Republican congressman Mike Garcia told Wray at the hearing on Thursday.

“I don’t think that this is necessarily a funding problem that we have for your agency as much as a leadership problem.”