The alleged assailant was not named, and it was not clear Saturday whether authorities planned to prosecute her as an adult.
McNair was the second D.C. high school student to be stabbed in the Metro system in recent days. On Tuesday, an 18-year-old male was stabbed as he stood on the platform of the Congress Heights station at about 3:30 p.m. The victim was hospitalized, but police said his wounds were not life-threatening. Police said in that incident the victim and the alleged assailant knew each other.
In the Friday attack, police said McNair was riding an Orange line train toward the Capitol South station when he was stabbed about 12:37 p.m. He stumbled toward the fare gates, apparently bleeding from the neck, as three Metro employees tried to help him.
A bystander’s video showed McNair lying in a fare gate as his friends screamed for help. Charles Decker, who was in the station, told reporters Friday that McNair was bleeding heavily and that a nurse who happened to be there and a Metro officer used towels to try to stanch the bleeding.
Police said the stabbing stemmed from a dispute between the boy and girl but did not provide further details. The police did not say where McNair attended high school. District public schools had the day off Friday, but charter and private schools in the city did not. McNair’s friends were wearing backpacks.
While McNair was rushed to a hospital, U.S. Capitol Police found and arrested the girl several blocks from the Metro station, D.C. police said. She was identified only as a resident of Northwest Washington and was charged with assault with intent to kill. The charge was amended to murder Saturday.
The Capitol South station was closed for more than five hours while District police investigated. The station, which is across the street from the Cannon House Office Building and near the U.S. Capitol grounds, reopened at 6 p.m.
McNair was the sixth person killed in the District since Wednesday. When the stabbing happened Friday, D.C. Mayor Muriel E. Bowser (D) and Police Chief Peter Newsham were announcing an anti-crime initiative, which includes adding extra police officers and stepping up social services in six high-crime areas of the city.
Newsham said targeting certain areas of the city had helped reduce homicides by 44 percent in those communities, along with significant reductions in shootings and other types of crime.
Peter Hermann, Paul Duggan and Justin Wm. Moyer contributed to this report.