A law enforcement officer walks on the street outside the home of Nicholas Young, a Washington Metro Transit Officer, Wednesday, Aug. 3, 2016, in Fairfax, Va., Young was arrested at Metro’s headquarters in Washington and charged with a single count of attempting to provided material support to a terrorist group. (AP Photo/Pablo Martinez Monsivais)

The Metro Transit Police officer accused of attempting to provide material support to ISIS was an average-to-below-average employee in the agency’s patrol bureau, whose performance issues never rose to the level of termination — even as an FBI probe was opened into his strange behavior, Metro officials said Thursday.

When Nicholas Young, of Fairfax, Va. began exhibiting “abnormal” and concerning behavior, the agency notified the FBI, Metro Transit Police Chief Ron Pavlik said. But while the federal monitoring took place, Young remained on the force, Pavlik said, until he was arrested Aug. 3 outside Metro headquarters; his employment terminated.

Young was jailed on terrorism charges after, court records said, he provided an undercover federal agent with mobile messaging cards, believing they would be used by Islamic State fighters to communicate overseas.

At the time of his dismissal, other law enforcement agents had been monitoring him in some capacity for seven years.

It was the first time a U.S. law enforcement officer had been accused of trying to provide support to a terrorist organization.

During the time he was monitored, however, Pavlik said Young never posed a threat to Metro employees or passengers.

“At any point in time if Mr. Young’s behavior escalated to the point where he was going to take action,” Pavlik said, “steps were in place to mitigate those.”

Even so, the arrest of a Transit Police officer on terrorism charges left Metro Board members dumbfounded. They wondered how someone with such dangerous inclinations could have been hired in the first place.

Upon his hiring in 2003, Young was subjected to a rigorous background check, including federal and local fingerprint scanning, a credit check, education and reference verification, and a truth verification exam similar to a polygraph test.

Chairman Jack Evans grilled Pavlik at a Thursday meeting of the transit agency’s board of directors.

“Was he in a position that he could have done destructive things?” Evans asked. “Why did we arrest him when we did? There’s a million questions.

“What happened here to the extent that you can tell me?”

Pavlik explained that Metro Transit Police contacted a local FBI field office when Young first exhibited the alarming behavior, the nature of which was not specified. At that point, he said, federal monitoring began.

As The Post previously reported, Young’s first contact with the FBI came in 2010. Later, he was in touch with an undercover FBI agent from Jan. 2011 to Feb. 2012. The probe stalled for a year but picked back up in 2014, leading up to Young’s arrest.

Metro General Manager Paul J. Wiedefeld was briefed on the case around the time he took over the agency in November.

In his briefing with the board, Pavlik attempted to explain why so much time elapsed between the opening and closing of the federal probe.

“The FBI, in our partnership, had means in place that if he posed an immediate risk to any of our employees or our riders that action would have been taken swiftly,” Pavlik said. “Unfortunately, investigations of this magnitude take a long period of time. We have all kinds of rules and laws that we have to adhere to. So it never goes as quick as we like but we’re happy that it ended the way it did.”