Reports of an active shooter at Walter Reed military hospital in Bethesda, Maryland on Tuesday were a "false alarm," officials tweeted.
Naval Support Activity of Bethesda tweeted that after investigating the origin of the active shooter report that caused the hospital to go on lockdown, officials "determined that this was a false alarm and not part of a scheduled drill as has been reported."
Army Lt. Col. Audricia Harris, a Pentagon spokesperson, previously told Fox News that the incident was part of a drill.
The U.S. Navy tweeted that upon further investigation, the incident was sparked due to "improper use of a mass notification system by a tenant command aboard the installation."
"While preparing for an upcoming drill, the notification system was inadvertently enacted ... without containing the words 'EXERCISE' or 'DRILL," the Navy wrote. "Individuals who saw the mass notification statement immediately notified NSA Bethesda security, where they responded accordingly and instituted an installation-wide active shooter response."
The "improper use of the system" was to blame, officials said.
After the ordeal, employees at the hospital, which is the "world's largest military medical center," tweeted confirmation there was no active shooting incident, and their Code White alarm, used to tell those within the hospital to shelter in place, was lifted.
Earlier Tuesday, Rep. Dutch Ruppersberger, D-Md., tweeted that he was "safe in a conference room w/ approx 40 others" at the hospital, "where we've been told there is an active shooter."
The congressman, whose Twitter account indicated at one point that he "does not believe this to be a drill," tweeted after the all clear that "at no point was there any indication that this was a drill."
The medical center had confirmed the building was on lockdown and that an alert was sent out about an active shooter. Naval Support Activity of Bethesda tweeted that security "has cleared the basement of Bldg. 19," which officials previously said was locked down.
"No indication so far of an active shooter," NSA Bethesda wrote.
Montgomery County police tweeted that they were called to Walter Reed at 2:23 p.m. "to assist with the report of a possible active shooter." The U.S. Navy also tweeted that officials were aware of the reports.
Tuesday's incident follows a string of false alarm claims of active shooters at numerous U.S. military bases across the country.
Fox News' Lucas Tomlinson, Jennifer Griffin and Jake Gibson contributed to this report.