Next month marks the 20th anniversary of the Long Island Rail Road massacre that killed 6 people and injured 19 more, and a new documentary about that deadly day is set to debut on Friday.

The 90-minute documentary, entitled "Long Island Rail Road Massacre," takes viewers back to Dec. 7, 1993. Gunman Colin Ferguson walked down the aisle of a crowded LIRR train and opened fire. It was deadliest mass shooting in Long Island history.

The film's director, Manhasset Hills native Charlie Minn, lost a classmate that night. He says it was a story that had to be told.

"I can only speak for the victims' families because I talked to them," Minn says. "They're forever altered, more psychologically than anything. Twenty years later, we are still asking the same tragic questions: how and why this could happen?"

There will be a special advanced screening of the film on Monday at Merrick Cinemas. It will open on Nov. 15 for a one-week run.

Ferguson was convicted of murder in 1995 and sentenced to six consecutive life sentences.