American University lifted a lockdown Wednesday evening almost two hours after a report of a man with a gun on campus about 7:40 p.m.
An AU student noticed a man with a gun in his holster on a university shuttle bus, took a picture, then called police, D.C. police said.
AU officials directed students to shelter in place.
The image was circulated on social media through AU's official account and helped generate leads and identify the individuals.
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Dozens of officers searched AU's campus for more than an hour until identifying the man with the gun as an off-duty D.C. police officer.
The lockdown was lifted around 9:30 p.m.
"Every student is safe tonight," AU spokeswoman Camille Lepre said in a statement. "In this case, someone saw something and said something. The system worked."
No weapon was ever displayed, no threats were made, no crime was committed and there were no arrests or injuries, Lt. Jesse Porter said.
"At this point, there's no crime at all," he said.