The 'gunman' who prompted a Princeton University lockdown turned out to be a teen with a fake gun, police say.

The 'gunman' was spotted walking around campus Wednesday morning. The campus was closed and school officials issued e-mail and text alerts telling students a gunman was on campus.

Police raced to the scene and arrested four teenagers they say they had the fake gun. Authorities say the teenagers found the fake gun while rummaging through a drop-off spot where graduating students dump items they no longer need.

The teenagers were not Princeton students, according to the university. The teens have been questioned and released. It is not clear yet if they will be charged.

Princeton says it's pleased the alert system worked well. A number of graduating seniors say they were never alerted, however, because the incident happened a day after graduation.

"Our system basically calculates who's on campus at what time and because we had our commencement exercises, seniors had actually been removed from the system. That is something we're going to look at, however," says Princeton's Cass Cliatt.