A photo circulating on social media over the weekend of a plane flying under a Delaware River bridge is real, according to the bridge’s owner.

The single-engine plane passed under the Riegelsville free bridge about 4:30 p.m. Friday, headed downstream under the span linking Riegelsville in Bucks County and Pohatcong Township in Warren County, the Delaware River Joint Toll Bridge Commission confirmed.

"That's a rather dangerous stunt and we're taking it very seriously," commission spokesman Joe Donnelly said Monday.

The commission alerted state police in both Pennsylvania and New Jersey, and the Federal Aviation Administration opened an investigation into the incident, Donnelly said.

Whether the pilot was breaking any laws is a question for the investigators, according to Donnelly.

The Intelligencer in Bucks County quotes FAA administrator Lee Abt as saying if the maneuver is not illegal, “It does not meet any kind of regulation that we would ever support."

A message left with Abt seeking comment was not immediately returned Monday.

Some social media comments about the photo suggested it was a remote-controlled plane, according to the website's report. One Facebook user swapped out the plane in favor of a T.I.E. fighter from "Star Wars."

But the bridge commission says it was a real plane, piloted by an unknown person.

"Whoever did fly it, it would probably be in their best interests to come forward to talk to the FAA," Donnelly said.

The deck of the Riegelsville bridge is 38 feet 3 inches above the low-water mark of the river there, according to the commission.

"The water's been running low," Donnelly said, "so that's pretty much in the ballpark of how much clearance there was."

Donnelly said he’s unaware of anyone trying a similar stunt in his 11 years with the bridge commission.

Kurt Bresswein may be reached at kbresswein@lehighvalleylive.com. Follow him on Twitter @KurtBresswein and Facebook. Find lehighvalleylive.com on Facebook.