Three people were injured — one critically — in two, unrelated fights aboard Metro trains Friday night, and authorities said Saturday that they were combing through surveillance footage, witness statements and physical evidence to determine what prompted the sudden spasm of violence.

The first incident was a fight between two men aboard a Green Line train traveling between the Greenbelt and Fort Totten stations. One of the men, 46, suffered a life-threatening head injury, authorities said. The second incident was a more sizable brawl that injured two teenagers — one stabbed in the back three times — but police said both teens were expected to survive.

The mayhem began between 9:30 p.m. and 9:45 p.m., authorities said, when two men got into a fistfight on a train headed to Fort Totten. Metro Transit Police boarded at the Fort Totten station and found a man with what authorities said were critical injuries.

Dan Stessel, a Metro spokesman, said that man was taken to a hospital with a head injury that was considered life-threatening.

Stessel said the other man in the fight was not at Fort Totten, but police later took him into custody at the L’Enfant Plaza station.

On Saturday, Metro Transit Police charged Dionte Daniels, 22, of Glenarden, Md., with attempted second-degree murder, assault and reckless endangerment in the fight.

It is unclear what prompted the dispute or whether the men knew each other, Stessel said. Metro released surveillance images of both men and urged anyone who may have witnessed the incident to call Metro Transit Police at 202-962-2121 and reference case 2014-11573.

He said investigators recovered no weapons aboard the train, which was taken out of service after the fight, causing minor delays.

The next fight — which Stessel said was unconnected to the first — broke out about 11:20 p.m. aboard a Blue Line train headed into the District. At the Stadium Armory station, Stessel said, a station manager called police, and the fight spilled from the train to the platform.

Stessel said the incident “appears to have started as a fight among young juveniles and young adults,” though he was not sure why they were fighting. A 19-year-old, he said, was stabbed in the back three times and hospitalized, and another teenager was treated at the scene with a cut on his hand and released to his mother, Stessel said.

Neither of the teens’ injuries was thought to be life-threatening, he said.

Stessel said investigators recovered a hunting knife with a three-inch blade at the scene and stopped several people outside the station who might have been involved in or witnessed the fight. He said that as of Saturday morning, the 19-year-old had said he did not want to pursue charges and was not cooperating with investigators. Metro authorities on Saturday released surveillance images of several people they say they want to identify in connection with the fracas.

Stessel said trains bypassed the Stadium Armory station after the brawl Friday night, and normal service resumed when Metro reopened at 7 a.m. Saturday.

Violence aboard Metro trains and in Metro stations is not unprecedented. In October, a juvenile was stabbed during a fight involving several people at the L’Enfant Plaza Metro station, and a few months later, a fight on a New Carrollton-bound Metro train between two men who knew each other resulted in a bullet hole in the window of the Orange Line train.

Some people took to Twitter to voice concerns after an altercation on a Metro train in June spilled onto the platform at the Woodley Park station. And residents who frequent the bars and restaurants in the District’s Chinatown neighborhood may recall a 2010 brawl that began at the Gallery Place station and involved as many as 70 people.

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