Search divers were expected to return near dawn on Wednesday to the waters surrounding the twisted ruins of a bridge knocked down in Baltimore Harbor by a faltering cargo ship, leaving six workers missing and presumed dead.

The disaster also forced the indefinite closure of the Port of Baltimore, one of the busiest on the U.S. Eastern Seaboard, and created a traffic quagmire for Baltimore and the surrounding region.

As the odds of their survival vanished, the search for the six workers was suspended on Tuesday evening, 18 hours after they were thrown from the fallen Francis Scott Key Bridge into the frigid waters at the mouth of the Patapsco River.

Maryland State Police and U.S. Coast Guard officials said diminished visibility and increasingly treacherous currents in the wreckage-strewn channel made continued search efforts on the river too risky to continue overnight.

Starting at 6 a.m. (1000 GMT) on Wednesday, “we’re hoping to put divers in the water and begin a more detailed search to do our very best to recover those six missing people,” state police Colonel Roland Butler told reporters late on Tuesday.

“We do not believe that we’re going to find any of these individuals alive,” Coast Guard Rear Admiral Shannon Gilreath said at the briefing.

Rescuers pulled two other workers from the water alive on Tuesday, and one of them was hospitalized. The six presumed to have perished included workers from Mexico, Guatemala and El Salvador, according to the Mexican Consulate in Washington.