WASHINGTON — A US police officer died on Tuesday following a shooting outside the Pentagon building, according to three law enforcement sources. While the exact circumstances of the shooting are still unclear, CNN previously reported that an officer had been shot in the incident. Pentagon Force Protection Agency Chief Woodrow Kusse will brief reporters on Tuesday afternoon with Pentagon press secretary John Kirby to provide more information The "shooting event" occurred on the bus platform on the Pentagon complex and prompted a lockdown of the building with no personnel allowed outside, according to a message that was sent to the Pentagon workforce by the Pentagon Force Protection Agency. The lockdown has since been lifted, the agency announced later Tuesday, tweeting: "The Pentagon has lifted the lockdown and has reopened. Corridor 2 and the Metro entrance remain closed. Corridor 3 is open for pedestrian traffic." The event occurred outside the building on the Metro Bus platform, which is a major entrance to the Pentagon used by thousands of personnel every day entering and leaving the building. The bus platform is used by multiple bus lines in the area.Earlier Tuesday, Pentagon Force Protection Agency spokesman Chris Layman would not comment on if there was a shooter involved or if people had been injured but Arlington Fire and EMS tweeted that they "did encounter multiple patients," while responding to an "active violence incident" in the area of the Pentagon Metro. The status of the patients was not immediately provided. Defense Secretary Lloyd Austin and Gen. Mark Milley, chairman of the Joint Chiefs, were attending a scheduled President"s Daily Brief and intelligence update meeting with President Joe Biden at the White House when the incident at the Pentagon occurred, according to a defense official. The area where the incident took place is "still very much an active scene" for investigators, the official said. Pentagon officials "will cooperate fully with local and federal authorities" the official added. Austin is staying away for now "out of respect" for the investigators needing to work. The reported shooting has halted mass transit to the massive complex. Metro trains and buses are now bypassing the area, according to the Washington Metropolitan Area Transit Authority. "We will continue to do so until we are advised it is safe," WMATA spokesperson Sherri Ly said. The Pentagon is serviced by dozens of bus routes and Metro"s blue and yellow rail lines. The station is the D.C. Metro system"s eighth busiest station this year with more than 2,400 passengers boarding each day, down from an average of 15,000 per day pre-pandemic. As of July 31, 185 police officers had been shot so far in 2021, 35 of whom were killed by gunfire, according to The National Fraternal Order of Police. — CNN