SEATTLE (AP) — The West Seattle Bridge needs additional support before contractors enter the 600-ft. arched span to complete scheduled repairs needed so the bridge can be reopened for traffic, city officials said.

Seattle Mayor Jenny Durkan announced the emergency bridge closure after a sudden acceleration of cracks within its hollow girders, the Seattle Times reported.

Photos from the Seattle Department of Transportation show cracks that lengthened 5 ft. (1.5 meters) since August, including 2 ft. (0.6 meters) of growth from March 6 to March 23.

Uneven weight loads played a role in the cracks because a bus lane was added to the then-six-lane crossing last decade, Transportation Director Sam Zimbabwe told The Times. Including traffic, 80 percent of the bridge's weight is the bridge itself, indicating the cracks likely have multiple causes, Zimbabwe said.

Durkan said she expects the bridge to be closed for months.

Matt Donahue, city roadway structures director, said recently that the bridge can still support its own weight, and he's not concerned about it sagging or collapsing.

"We're taking all measures to protect the integrity of the bridge," Donahue said, adding that engineering began recently to make a shoring plan. "We're looking at shoring as part of a medium-term repair program."

Since the bridge closed, the lower swing bridge is restricted to transit, emergency vehicles and freight, while other traffic must detour through the peninsula's southern hillsides and the Duwamish industrial area.