One Capitol Police officer was killed and another injured Friday when a man crashed his vehicle into them near the U.S. Capitol, an attack that once again put the city on edge as threats stemming from the deadly insurrection in January had started to wane.

The slain officer was identified as William “Billy” Evans, an 18-year veteran who a neighbor said was the father of two children. It was not immediately clear how he was fatally injured.

Acting Capitol Police chief Yogananda Pittman said that after the officers were struck, a man exited the vehicle with a knife and started lunging. She said at least one police officer opened fire, killing the attacker.

Several people familiar with the investigation identified the driver as Noah Green, who shared an apartment with his brother in Virginia. The brother said that Green, who was in his 20s, struggled with drug use and paranoia and that his family worried about his mental state.

Green was not known to either D.C. or Capitol police, according to authorities. “It does not appear to be terrorism-related,” acting D.C. police chief Robert J. Contee III told reporters.

The incident occurred shortly after 1 p.m. on Constitution Avenue near the Russell Senate Office Building, once again drawing hundreds of police officers to the Capitol and plunging the sprawling complex into lockdown, with people inside buildings told to stay away from windows and those outside told to “seek cover.”

Parishioners at St. Joseph’s, across the street from the Capitol, were told to stay put after the Rev. Bill Gurnee had bowed as the last act of services on Good Friday.

One Capitol police officer was killed and another injured on April 2 when a vehicle rammed into them near the U.S. Capitol. (The Washington Post)

Authorities lifted the lockdown about 3:15 p.m., though parts of Constitution Avenue and other streets remained closed. President Biden and House Speaker Nancy Pelosi (D-Calif.) ordered flags at the White House and at the Capitol to be flown at half-staff. Evans is the sixth member of the Capitol Police force to die in the line of duty.

“Today, America’s heart has been broken by the tragic and heroic death of one of our Capitol Police heroes,” Pelosi said in a statement, calling Evans “a martyr for our democracy.”

Biden said in a statement that he was being briefed on the investigation by a Homeland Security adviser. He and first lady Jill Biden said they were “heartbroken” by the attack and Evans’s death.

The attack comes less than three months after the Jan. 6 insurrection that left five people dead, including Capitol Police officer Brian D. Sicknick. Two police officers who had been at the riot later died by suicide.

In a statement, Pelosi added that officers once again “risked their lives to protect our Capitol and our Country, with the same extraordinary selflessness and spirit of service seen on January 6.”

“I’m heartbroken for the officer killed today defending our Capitol and for his family,” said Senate Majority Leader Charles E. Schumer (D-N.Y.). “I’m praying for the officer injured and his family. We’re in their debt.”

The Capitol Police Labor Committee called the circumstances of Evans’s death a “catastrophic example of the risks” officers take “putting themselves in harm’s way to protect our Capitol, its occupants, and the seat of democracy itself.”

After Friday’s incident, the D.C. National Guard activated its new “immediate response force,” a group of troops who already were on Capitol grounds as part of a 2,300-member deployment for added security. On Friday, two dozen National Guard members formed a line across Independence Avenue, wearing helmets and body armor and carrying plastic shields.

A blue sedan remained at the scene, with its taillights flashing and trunk open, surrounded by crime-scene tape, near the temporary fence and at a permanent police checkpoint.

At a news conference outside the Capitol, Pittman said the driver of the vehicle entered what is called the North Barricade “and rammed into two officers” before hitting a barrier.

Steve Brown, a 60-year-old high school physics teacher at Herndon High School in Virginia, said he was driving on Constitution Avenue and saw the vehicle strike the officers.

He said he parked, got out and walked toward the barricade. He said the attacker got out of the blue vehicle with a knife in his right hand. He said the knife appeared to be at least 10 inches long.

Brown said that the man walked toward him and that he backed up. He said the attacker turned and headed back toward the barricade, where he was shot.

Pittman said the man did not respond to verbal commands before the officers fired.

Both injured officers were taken to hospitals, where Evans died. Capitol Police said the second officer’s injuries were believed to be non-life-threatening. “Please keep the U.S. Capitol Police in your prayers,” Pittman said.

The driver, Green, was a onetime defensive back on Christopher Newport University’s football team who later moved to Indianapolis, and then to Botswana, before calling his brother in Virginia seeking help, his brother said.

His brother said Green had suffered hallucinations, heart palpitations, headaches and suicidal thoughts that could have been related to drugs or mental illness. Green wrote on his Facebook page that he had embarked on a spiritual journey and that “these past few years have been tough, and these past few months have been tougher.”

On his Facebook page, Green listed himself as a “Follower of Farrakhan” — an apparent reference to Louis Farrakhan, leader of the black nationalist group Nation of Islam. His last post links to a video by the Nation of Islam leader Elijah Muhammad that Green said was a “divine warning to us all during these last days of our world as we know it.”

Although the Capitol was largely empty because both chambers were on recess, a number of congressional staffers were on or near the campus as the incident unfolded.

One congressional aide, who spoke on the condition of anonymity because he did not have permission to speak publicly, said he was walking to the Longworth House Office Building to get his first coronavirus vaccine shot when his phone buzzed. He could hear the sirens.

“If you are outside, seek cover,” the alert warned.

“So I ran to my car,” he said.

It was supposed to be his first time back to his office since the Jan. 6 insurrection.

Ralph Jones, communications director for Rep. A. Donald McEachin (D-Va.), had also chosen Friday as his first day back in the building since Jan. 6. He came to renew his security badge allowing access to buildings in the complex. Fifteen minutes after he left, as he sat on a Metro train headed home, the urgent warning popped up on his phone, too.

He decided he would not be returning to the Capitol until he was mandated to do so.

“I squeaked out just in time, and I have no plans to go back any time soon,” he said. “Really there’s nothing stopping me from going except that home still feels much safer to me than being in the Capitol, and it has nothing to do with covid-19.”

Capitol Hill residents and tourists came by to pay respects to the fallen officer and try to see the scene for themselves.

Grace Streit, 47, and her 19-year-old daughter, Julia, had been at the Capitol for the Biden inauguration and saw the intensity of the security protocols that followed the January riot.

“I was shocked that anything like this could still happen. It felt like things had really died down,” Streit said. “Now we are here for more tragedy? More attacks on the Capitol? We had hoped this was behind us.”

Clarence Williams, Aaron C. Davis, Devlin Barrett, Mike DeBonis, Meagan Flynn, Justin Jouvenal, Julie Tate, Dan Lamothe, Donna Cassata, Paul Duggan, Paul Kane and Rachel Weiner contributed to this report.