The mystery drones detected flying over New Jersey in recent weeks are “concerning” and the FBI does not know who is operating them, a senior bureau official told Congress on Tuesday.

Robert Wheeler Jr., who runs the FBI’s critical incident response group, said they don’t see any evidence yet to suggest nefarious motives but they haven’t been able to rule that out, either.

He said the drones are in some cases operating over what the government deems sensitive sites.

“It is concerning,” he told the House Homeland Security Committee.

The drones have sparked curiosity — and some panic — in New Jersey, where dozens of reports are being made each night.

The lack of answers, particularly about drones over military bases in the area, had lawmakers miffed.

“Why are we not taking action?” demanded Rep. August Pfluger, the Texas Republican who convened Tuesday’s hearing on drone dangers.

Mr. Wheeler offered a bureaucratically particular response, saying that “authority exists to mitigate” a drone while in flight, but not saying why it hadn’t happened yet.

New Jersey Gov. Phil Murphy this week called the drones “sophisticated” and said they “go dark” once they are detected.

Even with eyes on New Jersey, experts said there are ongoing threats elsewhere.

At the U.S.-Mexico border, Customs and Border Protection said during a recent six-week period they detected 6,900 flights near the boundary. Keith Jones, a top official at CBP, said they can’t be certain of the intent of each of them but said a “large percent” are nefarious.

He said the risk is “increasingly rapidly.”

“We’re already seeing drug cartels in Mexico use drones to attack the military, police and their rivals,” Mr. Jones said.

For the U.S., he said those are potential future dangers, but the more current threat is that smuggling operations use drones as surveillance platforms. They track U.S. assets and spot holes in border defenses, so they know when to send over high-risk contraband.

• Stephen Dinan can be reached at sdinan@washingtontimes.com.