Two workers are presumed dead after a water main broke in the South End today, and a recovery effort is under way in the deep trench, according to the Boston police and fire departments.

"It looks like a pipe may have broke and unfortunately they weren't able to get themselves out of the hole," Boston Police Commissioner William Evans said. "Right now the most important thing we are trying to do is recover the bodies."

A tech rescue response team is on scene at 10 Dartmouth Street, which has been closed, and the water and power have been shut off in the area. Authorities are urging commuters to avoid the neighborhood.

Boston Fire Chief John Walsh said workers caught in the trench are not city workers, but private contractors.

"I think the hole is reported to be around 12 feet. As they drain it we will get a better idea how deep it is," he said. "They were out here working on the water system and we don't know what happened after that."

The scene was chaotic immediately after the incident, with workers shouting to each other to try to move large metal plates.

Firefighters desperately tried to get access to the trench, racing to the scene and reaching through the rushing water. One worker appeared to emerge from the trench, visibly shaken, holding his hand to his forehead as the knee-high water rushed by him.

Stunned residents of Dartmouth Street observed the scene, with one woman holding her hands to her head saying, "oh no, oh no."

"Response to area 10 Dartmouth St. for a water main break flooding a 2'x20' trench in the street," the fire department tweeted at 2:14 p.m. "At least 1 worker trapped."

The fire department is using a large vacuum to drain the water out of the hole — "unknown how deep," the department said in a tweet.

Once most of the water is out of the hole, a trench box will be lowered to secure walls of the trench to prevent collapse. Then rescue workers will enter the trench to begin the recovery of the bodies.

Boston Mayor Martin J. Walsh says he expects the area to be closed to traffic for the next few days.

"My thoughts and prayers are with these two people's families and everyone's family. It's a tough situation," said the mayor, a former construction worker. "Construction is a dangerous job and it's just a tough situation."

Evans said the recovery is expected to last through the night into tomorrow morning and added that there would be a police investigation.

"We brought in our homicide unit as we would with any accident like this. We will work with OSHA, we will work with the fire department," Evans said.

He said at this point he does not suspect any foul play.

"It looks like an accident."

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