BOSTON—New details emerged Sunday of the accused marathon bombers' alleged theft of a sport-utility vehicle before a chaotic gunfight in the city's suburbs.

About 40 minutes after they allegedly shot and killed a Massachusetts Institute of Technology police officer in Cambridge, Mass., Tamerlan and Dzhokhar Tsarnaev, the brothers accused of the bombing, crossed the Charles River into Boston and stole a Mercedes SUV at gunpoint, briefly holding the driver hostage, according to an excerpt from the Cambridge Police Department report filed by the driver and reviewed by The Wall Street Journal. The alleged carjacking occurred at about 11 p.m. in Boston's Brighton neighborhood on Brighton Avenue, said the excerpt, which didn't name the victim.

According to the report, one of the brothers jumped out of a Honda Civic and reached in through the front passenger side window of the SUV, opened the door and pointed a silver colored semi-automatic pistol at the driver.

The gunman then ordered him to drive to Watertown and was followed by the Honda, the report said.

Once in Watertown, the victim was ordered to slide over to the passenger side, the report said. The two brothers then removed luggage from the Honda and loaded it into the trunk of the Mercedes, the report said.