An active shooter alert at the naval facility that is home to Walter Reed National Military Medical Center near Washington, D.C., turned out to be a drill that someone forgot to tell anyone about, the Navy said Tuesday.

The Navy said someone at Naval Support Activity Bethesda, in the Maryland suburbs of the capital, was planning a future drill when a tenant command "inadvertently" activated the facility's alert system at about 2 p.m. ET without including the words "exercise" or "drill."

Patients at Walter Reed, along with staff and visitors at the naval facility, scrambled for safety as security officers and Montgomery County police converged in response.

Authorities eventually determined that an "improper use of the system was the root cause" and issued the all-clear, the Navy said.

The Navy didn't identify which of the facility's two dozen tenant commands — most, but not all, of them directly affiliated with Walter Reed — was responsible. But it said the false "active shooter" was reported to have been in Building 19, near the main medical center tower and patient parking.

Among those who were locked down was Rep. C.A. Dutch Ruppersberger, D-Md., who live-tweeted the drama, saying he and dozens of other people were sheltered in a single room.

"At no point was there any indication that this was a drill," he tweeted.

I am currently at Walter Reed Medical in Bethesda where we've been told there is an active shooter. I am currently safe in a conference room w/ approx 40 others. — Dutch Ruppersberger (@Call_Me_Dutch) November 27, 2018

We've been given the all clear at Walter Reed - at no point was there any indication that this was a drill. — Dutch Ruppersberger (@Call_Me_Dutch) November 27, 2018

Ruppersberger said later that he would be "following up to see what went wrong and how the process can be improved for the future."

Local police said that it was common for institutions like NSA Bethesda to let them know in advance about exercises but that they were completely in the dark about Tuesday's accidental "drill."