WASHINGTON – Prosecutors say a three-man Russian crew may have left the bridge unmanned on a tanker alleged to have rammed a fishing boat and killed three fishermen off the New England coast last summer.

Details of the developing U.S. case against the three Russians, each of whom is charged with misconduct and involuntary manslaughter, are included in U.S. District Court records filed to support the sailors’ extradition from Canada.

If the case goes to trial, prosecutors are prepared to argue the Russians ignored international maritime rules requiring ships to maintain a lookout at all times, the court documents indicate.

Investigators believe the crew either left the bridge unmanned so they could clean tanks to prepare to take on a load of gasoline or failed to monitor any of the ship’s radar or communications systems.

“It is virtually impossible for any responsible seaman, who was even minimally attentive to his duties, to have failed to recognize the impending collision,” Michael J. Keegan, the U.S. Justice Department’s trial attorney, wrote in papers filed in support of extradition.

The Starbound, an 83-foot trawler based in Rockland, Maine, was about 160 miles off the Massachusetts coast and headed for home early on Aug. 5 when it encountered the Virgo, a 541-foot Cypriot-flagged tanker with the Russian crew. Ship’s logs indicate the Virgo crew was involved in tank cleaning during the early morning hours that day, but don’t pinpoint times.

Joseph Marcantonio of Gloucester, Mass., the Starbound’s captain and its only survivor, has said he was awakened by the shouts of the crewman on watch, James Sanfilippo of Thomaston, Maine. Marcantonio reached the wheelhouse in time to see “the towering bulbous bow of a large vessel” bearing down on his boat, according to the case record filed in support of the extradition.

The Virgo struck the port bow of the Starbound, sinking it within minutes, according to the Coast Guard. The Virgo never stopped and didn’t respond to repeated attempts by the Coast Guard to make contact, records show.

Marcantonio struggled into a life raft and was rescued by a boat that had been dropping lobster traps in the area. Sanfilippo’s body was recovered, but those of the other two crewmen, Mark Doughty of Yarmouth, Maine, and Thomas Frontiero of Gloucester, are presumed lost at sea.

Two days later, the Virgo – bearing teal green paint scrapings that U.S. authorities say match the Starbound’s custom-mixed paint – was stopped by Canadian authorities. A week later, the ship’s captain, Vladimir Ivanov, and crewmen Dmitriy Bogdanov and Mikhail Gerasimenko, were arrested at a Canadian airport as they tried to leave for home.