WOODBURY - Friends and family are mourning a Nesconset teen who was fatally struck by a New York City subway yesterday evening while out celebrating his 18th birthday.

Police say Liam Armstrong, a senior at Smithtown High School East, was killed by an uptown 2 train when he and his friends tried to cross four sets of tracks to get to the downtown platform at the 79th Street Station.

Friends say Armstrong played for the school's lacrosse team until his junior year and had just joined a local soccer club. They say he had already enlisted in the Marine Corps and was set to leave in November.

The flag outside Smithtown High School East now flies at half-staff in memory of the teen, who was set to graduate in May.

In a message on the school district's website, the superintendent called Armstrong "a popular student among his peers and the high school staff. He was a vibrant young man who will be missed by all who knew him."

News 12 Long Island has learned that Armstrong's father is a detective sergeant with the Nassau County Police Department. The family has asked for privacy.