Police were hampered at the Washington Navy Yard shooting in September 2013 because a security officer had locked a video surveillance room, which prevented officers from making a clear assessment of the situation, according to a department report released Friday.

As a result, the authorities issued a public warning that an additional shooter could have been involved, and a portion of the city was locked down on September 16, The New York Times reports.

“Security camera footage would allow responders to review and confirm various details and events as they unfolded and the investigation progressed,” according to the 83-page report. “The footage may have also allowed police to quickly identify the shooter, ascertain his movements and help in determining whether others may have been involved.”

District of Columbia police also had trouble entering the gated facility, finding the correct building and navigating the layout as they looked for the shooter. Navy Yard practices also may have led to some easy confusion, as witnesses called the internal emergency number rather than 911.