It was not immediately clear how the officer was fatally injured.
The incident that occurred about 1 p.m. comes less than three months after the Jan. 6 insurrection at the Capitol left five people dead, including Capitol Police Officer Brian D. Sicknick. Two police officers who had been at the riot later died by suicide.
It also comes two weeks after law enforcement authorities removed an outer perimeter fence, opening Independence and Constitution avenues to traffic. An inner-perimeter fence remains around the complex.
On Friday, all buildings on the U.S. Capitol complex were put on lockdown. Officials said Attorney General Merrick Garland has been briefed on the incident and is monitoring the investigation.
The FBI said in a statement that its Washington field office is responding and providing support to Capitol Police.
The blocks near the Capitol building were once again filled with law enforcement as the area was secured. Just before 2 p.m., two dozen National Guard troops formed a line across Independence Avenue, wearing helmets and body armor and carrying plastic shields.
Vehicles and pedestrians were blocked from going east on Constitution Avenue NE by barricades and armed National Guard members, along with D.C. police patrol cars.
The U.S. Capitol Police said on Twitter that the incident occurred at an access point along Constitution Avenue.
When the incident occurred, police alerted staff members: “Due to an external security threat located All U.S. Capitol Campus Buildings, no entry or exit is permitted at this time,” the alert read. “You may move throughout the building(s) but stay away from exterior windows and doors. If you are outside, seek cover.”
Jessica Contrera, Devlin Barrett, Michelle Boorstein, Justin Jouvenal and Paul Kane contributed to this report.
