The FBI is pursuing 1,000 investigations into suspected “lone wolf” militants and another 1,000 into “domestic terrorists”, the bureau’s director has told a congressional committee.

Christopher Wray said so-called “lone wolf” terrorists – whom another law enforcement official described as individuals often radicalised over the internet or other social media – are the FBI’s “highest counterterrorism priority”.

Speaking at a Senate appropriations hearing, Wray said the FBI had about 1,000 investigations into suspected lone wolves in all 50 states, “and that’s not even counting the al-Qaida investigations, the traditional [Islamic State] investigations, the domestic terrorism investigations”.

“And what makes it so hard is that there are not many dots to connect with some of these people,” he said. “They pick soft targets, they use easy-to-use weapons; you know, IEDs (improvised explosive devices), cars, knives, guns.”

Several mass shootings in recent years have been attributed to so-called “lone wolves” whose activities are difficult to predict, such as Orlando shooter Omar Mateen.

Wray said the FBI was “trying to get better at looking for red flags” that could signal when people becoming radicalised might start to consider taking action.

In addition, Wray said, the FBI was pursuing another 1,000 investigations into “domestic terrorists”.

The second law enforcement official, who spoke on the condition of anonymity, said rightwing extremists, violent animal rights and anti-abortion extremists, and African-American or left-wing militants fell into this second category.

There was an “overlap” in FBI statistics, the official said, on the number of lone wolf investigations and investigations related to the Islamic State movement.

Patrick Leahy, a senior Democratic member of the appropriations committee, said that as part of the annual budget request the FBI had presented, Donald Trump’s administration reduced the FBI budget by 5%, rescinded planned allocations of $148m in salaries and expenses, and dropped or at least postponed plans for a new FBI headquarters.