A routine safety inspection was mistaken for an active shooter situation at Joint Base Andrews on June 29 around 9 a.m., prompting emergency responders to flood onto the base. A sweep of the facility was conducted and it re-opened later that day. (WUSA9)

A routine safety inspection was mistaken for an active shooter situation at Joint Base Andrews on June 29 around 9 a.m., prompting emergency responders to flood onto the base. A sweep of the facility was conducted and it re-opened later that day. (WUSA9)

Someone mistook a routine safety inspection for an active shooter situation at Joint Base Andrews on Thursday morning, prompting tense moments and a lockdown of the sprawling facility in Prince George’s County, Md., base officials said.

The base, which is home to Air Force One and thousands of military personnel, was briefly sent into confusion. Law enforcement streamed across the facility, cable news carried live updates on the incident, and a trip from the base by Vice President Biden was delayed.

The person reported the active shooter situation at about 9 a.m., after misidentifying security forces who were conducting a routine inspection of the Malcolm Grow medical facility as a potential threat, the officials said. Officials would not say whether the security team was armed.

The base sent out an order to shelter in place a short time later.

Authorities could be seen moving across the campus in black SUVs and later evacuating people from the medical facility. One person who works at the base said he received an email that told employees to remain locked in their offices and read: “This is a real-world event. Active shooter.”

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The incident came two days after a terrorist attack in Istanbul and a couple of weeks after the nation’s worst mass shooting in Orlando, at a time when the nation is on edge.

“Heightened tensions nowadays mean everyone is very vigilant,” said Maj. Jaime Davis, a Defense Department spokesman. “We don’t want people to not say anything if they see something, but obviously there needs to be clarity of what is being seen and said.”

The situation was further confused because the base was scheduled to conduct a “no notice” active shooter drill in the late morning on the opposite side of the campus, which is about 14 miles from downtown Washington, the base said.

Officials said the drill and the initial report were not linked.

By 10:40 a.m., a sweep of the facility had been completed and authorities gave the base an “all clear.” The Grow building remained on lockdown after the general lockdown was lifted, but authorities determined there was no threat to the base.

“Fortunately, this was not a life-threatening situation,” Col. Brad Hoagland, the base commander, said in a statement. “We take all threats seriously and reacted to ensure the security of those on the base.”

A retired Navy commander, who declined to be identified, said he arrived shortly before 8 a.m. to the third floor of the medical center for an exam. He was initially told that a drill was planned, but then staff arrived with news that there could be a real emergency.

“One of the medical staff members came in and said we have a real-world event and we would like you to switch rooms,” the retired officer said. “There were nine of us. They took us to a room and they locked the door. I felt pretty safe.”

The officer said the group stayed in the room for about an hour and a half.

Afterward, the Rev. Aaron Harley, pastor of Redemption Ministries in Temple Hills, was among those gathered at the McDonald’s at the front gate of Andrews.

“I’m just glad that it wasn’t real,” he said. “As pastors, we have to do the funerals, we have to do the consoling, and these incidents are serious. There is just too much killing.”

Joint Base Andrews is a combined Navy and Air Force base.

Air Force One landed at Andrews on Wednesday night, bringing President Obama home from a trip to Canada. According to a White House pool report, the president deplaned at 9:15 p.m. and “stepped aboard Marine One a moment later.”

Last month, the base was put on lockdown after a woman walked into the visitors center and said she had a bomb strapped to her chest. The incident turned out to be a hoax, and the base was reopened after about 90 minutes.

Matt Zapotosky, Mark Berman, Amanda Finnegan, Peter Hermann, Thomas Gibbons-Neff and Josh Freedom du Lac contributed to this report.