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.
Jessica Simpson shares three-way kiss with friends in photo
Scarlett Johansson steps out with fiance after pregnancy reveal