A chemical leak near the Delaware Memorial Bridge closed the dual span for more than 6 ½ hours on Sunday, one of the busiest travel days of the year, causing backups and snarling traffic on both sides of the river.

The Delaware River and Bay Authority said the twin spans were closed due to a "gas leak" about 4 p.m., at the height of the post-Thanksgiving rush.

The leak occurred at the Croda chemical production facility at Atlas Point, located near the bridge on a major route between Washington and New York City, said George Greenley, public information officer for the Holloway Terrace Fire Company in New Castle.

Police on the scene began allowing motorists back onto the bridge around 11:30 p.m. Sunday.

The leaked chemical was ethylene oxide, a highly flammable gas that is a finished product stemming from methanol, Greenley said.

"If that flame would have had an ignition source, it could have been catastrophic with the bridge traffic," he said. The Wilmington News Journal reported that police were using reverse 911 to tell residents in New Castle to stay in their homes. The Delaware River and Bay Authority, which maintains the bridge, and New Castle County police had no immediate reports of injuries.

Operators of the 140-acre Croda site requested the bridge be shut down, the Delaware River and Bay Authority tweeted.

Greenley said fire officials were waiting for pressure to dissipate in the pipes at the facility before reopening the bridge. He said the chemical was dissipating fairly quickly from the pipes. The state Division of Natural Resources and Environmental Control also was on scene at the leak.

Travelers were being warned to avoid the bridge.

The Delaware River and Bay Authority estimates that up to 90,000 motorists use the bridge daily.

If you were stuck on the bridge, we would like to hear from you. Send an email to citydesk@phillynews.com.

Southbound traffic on the New Jersey Turnpike backed up couple miles before Exit 2 as people need to exit there because Delaware Memorial Bridge closed on November 25, 2018.

Editor's note: This story was revised to reflect that the bridge was closed about 5 p.m., not 4 p.m.