WASHINGTON (WJZ) — High alert. A massive police response at the U.S. Naval Yard in Washington, D.C. following reports of gunfire. It comes less than two years after a man opened fire at the base, killing several people.

Although there was no gunfire this time, WJZ Investigator Mike Hellgren explains why the scare had many in Washington very concerned.

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People were terrified, fearing they were once again trapped in a building with a mad gunman. Thankfully, that was not the case. Police stress though they take every threat seriously.

Police flooded the Navy Yard around 7:30 a.m. Thursday after an employee called 911 and reported hearing gunshots. In the tense moments that followed, people ran for cover.

“We heard someone scream, ‘Get out of the building. Stay away from the cafeteria.’ And we saw everyone running for the exits or adjoining offices,” said Commander Scott Williams, U.S. Navy.

“Especially what happened a year and a half ago at the Navy Yard–you hope this wouldn’t happen again. But all of the sudden, I got a call this morning saying there was an active shooter again at the Navy Yard. And my heart stopped again because my son works there,” said Kevin Hawkins.

While an exhaustive search turned up nothing, police interviewed the caller and believe the call was legitimate.

“The person who made the call heard what she thought may have been gunshots. And she made a call. Which is what employees here are trained to do. We have no concerns that this was a hoax whatsoever,” said Chief Cathy Lanier, D.C. Metropolitan Police.

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M Street outside the Navy Yard is now back to normal.

This was especially scary for those who lived through a mass shooting there almost two years ago. Still chilling images show gunman Aaron Alexis roaming the halls of Building 197, looking for his next target.

Twelve people died that day. Authorities evacuated the same building this time.

“Things that we could have improved upon that we learned over the last couple of years, we’ve done exactly that,” said D.C. Mayor Muriel Bowser.

Police searched building after building there before they gave the “all clear” and say they remain vigilant on heightened alert for threats leading into the July 4 holiday.

“We do take every call seriously. And when we heard a call go out for a potential active shooter, we brought the resources that needed to be here,” said Chief Lanier.

The FBI and Homeland Security sent out an alert, urging local law enforcement to be prepared and vigilant for any terror threats, although they did not mention any active plots.

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In a preventative measure, D.C. Metro Police closed Pennsylvania Avenue in front of the White House.