Princeton University says the sound of a hammer hitting a chisel while a campus building's door was being fixed likely is what was reported as gunshots this week.

Tuesday night's report caused police to cordon off the New Jersey Ivy League university's Nassau Hall, an administrative center. University officials said hours later the report was unfounded.

Princeton's Department of Public Safety said Thursday it learned during a follow-up investigation a maintenance worker had been using the hammer and chisel 20 minutes before another worker at the building reported hearing gunshots.

Public Safety boss Paul Ominsky says the university takes all reports seriously and people on campus are encouraged to report concerns following the U.S. Department of Homeland Security's advice: If you see something, say something.