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An explosion struck a US Air Force recruiting office in northeast Oklahoma — the second time a recruitment center in the area was attacked in recent days, the FBI said.

Bomb squads responded to the explosion in Bixby, a suburb of Tulsa, at about 10:30 p.m. local time Monday, Bixby police Sgt. Tim Scrivner told CNN. No one was inside the Air Force recruitment center.

Now the FBI and the Bureau for Alcohol, Tobacco, Firearms and Explosives are investigating what type of bomb was used — and what the motive was.

Early reports indicate the fragments were consistent with a pipe bomb, FBI spokeswoman Jessie Rice said, but the type of bomb has not been confirmed.

A witness reported that someone on a motorcycle was in the area, and agents are pursuing that lead as they conduct interviews, Rice said.

The FBI is not calling the case an act of domestic terrorism because investigators don’t have a suspect or a motive. But terrorism has not been ruled out as a possible motive.

A few days earlier, a nearby National Guard recruiting center was vandalized, police said, according to the FBI spokeswoman. That vandalism included slashed tires.

The FBI said it is investigating whether the two attacks at military recruitment centers are related.

Police have cleared the scene in Bixby where an explosion went off around 10:30 last night. pic.twitter.com/1s50JHfrJc — Mycah Hatfield (@MycahABC13) July 11, 2017

Here's a look at the damage to the US Air Force Recruiting Center in Bixby from the explosion. The front door is lying in the parking lot. pic.twitter.com/M9nx7VA8TK — Mycah Hatfield (@MycahABC13) July 11, 2017