The school sent a message on Twitter saying one person was in custody.
The university's alert system said "a citizen reported to campus police that they saw a man with a gun" and the school tweeted that a lockdown had been ordered and students were told to shelter in place shortly before 8 p.m. EST.
University and District police posted messages on Twitter about 9:20 p.m. that the lockdown was lifted and no students were injured.
Authorities were investigating near Gray Hall, WUSA-TV, Washington, reported. Campus police tweeted a photo of two people, urging anyone had seen them to call police. The TV station said it was unclear what connection the pair had to the lockdown.