He supported Donald Trump. She supported Hillary Clinton.

But in the midst of the 2016 election, Brian D. Sicknick, an officer with the U.S. Capitol Police, and Caroline Behringer, an adviser for a liberal congresswoman, found common ground.

They met mornings at an entrance to the Capitol, she heading to her office, he protecting those doing the people’s work. They chatted about unwinding in the outdoors and joked about being on opposite sides of the political divide tearing the nation apart.

“There was a shared humanity,” Behringer said, noting, “My job was very much dependent on him keeping me safe.”

Sicknick, a 12-year veteran, died Thursday night, a day after police said he physically engaged with the riotous mob that broke into the Capitol trying to overturn the November election President Trump had lost.

The 42-year-old from South River, N.J., was the sixth U.S. Capitol Police officer to die in the line of duty since 1952, and the fourth to be a victim of an attack on the Capitol grounds.

Police did not provide details of how Sicknick was injured. His cause of death has not been determined, though homicide detectives from the D.C. police department have taken charge of the investigation.

Acting attorney general Jeffrey A. Rosen said in a statement that Sicknick died of “the injuries he suffered defending the U.S. Capitol, against the violent mob who stormed it on January 6th.” Rosen added the FBI and D.C. police “will jointly investigate the case and the Department of Justice will spare no resources in investigating and holding accountable those responsible.”

In a statement, Sicknick’s family said “many details regarding Wednesday’s events and the direct causes of Brian’s injuries remain unknown and our family asks the public and the press to respect our wishes in not making Brian’s passing a political issue.”

The statement added, “Brian is a hero and that is what we would like people to remember.”

Trump, who has often boasted of his support for law enforcement, did not publicly acknowledge Sicknick’s death in the hours after it was announced. The White House issued a statement Thursday, before the officer died, saying it “grieves the loss of life” at the Capitol and prays for the recovery of others. Three people died in what police have called “medical emergencies” and one woman was fatally shot by a Capitol Police officer.

In response to a query on Friday evening, a White House spokesman said Trump and his administration “extend our prayers to Capitol Police Officer Brian Sicknick’s family as we all grieve the loss of this American hero.”

On Friday, House Speaker Nancy Pelosi (D-Calif.) ordered flags at the U.S. Capitol to be flown at half-staff. Thursday night, officers lined up in front of the Capitol in silence to honor their fallen colleague.

“The violent and deadly act of insurrection targeting the Capitol, our temple of American Democracy, and its workers was a profound tragedy and stain on our nation’s history,” Pelosi said in a statement. “But because of the heroism of our first responders and the determination of the Congress, we were not, and we will never be, diverted from our duty to the Constitution and the American people.”

A family statement says Sicknick was the youngest of three brothers who grew up in a borough along the I-95 corridor south of New Brunswick, and earned a bachelor’s degree in criminal justice from the University of Phoenix. He rescued dachshunds and loved the New Jersey Devils hockey team.

He is survived by his parents, Charles and Gladys Sicknick, brothers Ken and Craig, and his girlfriend of 11 years, Sandra Garza. Relatives and close friends did not speak publicly on Friday.

Sicknick’s family said he had wanted to be a police officer his entire life. One of those brothers, Ken Sicknick, said in the statement that his sibling had joined the New Jersey Air National Guard “as a means to that end” following the 9/11 terrorist attacks.

He had been assigned to the 108th Air Refueling Wing out of Joint Base McGuire-Dix-Lakehurst in New Jersey. The New Jersey National Guard said it “was saddened by the loss” and said Sicknick joined in 1997 and deployed to Saudia Arabia in 1999 and to Kyrgyzstan in 2003.

Chief Master Sgt. Lance C. Endee, who was Sicknick’s squad leader, said that after 18-hour days, Sicknick would be the last to turn in his weapon, and on below-freezing nights at a bare-bones airfield in Kyrgyzstan, he would cheer up the rest of the squad.

“Brian always had a smile on his face — you know — nothing shook him,” Endee said. “Things would be going terribly wrong, everybody would be miserable and Brian would be able to joke about it.”

When praised for “going above and beyond,” he said Sicknick would “brush it off like it was no big deal,” saying “That’s what we do.”

The two stayed in touch for nearly two decades, sending jokes to their alumni group. When the news came last night, Endee said he heard from over three dozen current or former squad members hoping it was not the Brian Sicknick they knew. “I think Brian had a bigger impact on people than he would have ever realized,” Endee said.

Sicknick joined the Capitol Police force in 2008 and most recently served in the first responders unit.

Capitol Police officers are reeling, mourning the loss of a trusted colleague even as they try to comprehend the security failure that allowed the building they are charged with protecting to fall amid a riot.

“Everybody’s upset and shaken right now,” one officer, who like others spoke on the condition of anonymity because he was not authorized to speak publicly, said Friday outside of the Capitol Police headquarters. “There’s a lot of confusion. We’re tired and on edge and there’s just the feeling of ‘what’s next, what’s coming?’ ”

Another officer, who has been on the force 19 years, described members as “angry and frustrated,” adding, “Things have to change. We have a job to do and we can’t do it. And the change has to come from the chief on down.”

Sicknick was the first Capitol police officer to die in an attack since the summer of 1998 when Russell E. Weston Jr. walked into a public entrance of the Capitol and opened fire after passing through the metal detector. Officer Jacob J. Chestnut, 58, who had been writing directions for two tourists, was shot in the back of the head at point-blank range. When Weston then entered the office of Rep. Tom Delay (R-Tex.), Special Agent John M. Gibson, 42, exchanged fire with Weston and was fatally wounded. Weston was struck four times and was resuscitated by Sen. Bill Frist (R-Tenn.), a heart surgeon.

Those who encountered Sicknick said his political views did not align neatly with one political party.

“He was conservative, but polite and measured” in messages he sent to the office of his congressman, Rep. Don Beyer (D-Va.), according to the lawmaker’s spokesman, Aaron Fritschner.

He said Sicknick supported the president and opposed impeachment, but favored gun control. He was concerned about animal cruelty and the national debt.

Michael Ricci, who was a staffer to former House speakers Paul D. Ryan and John A. Boehner, said Sicknick was one of a half-dozen officers he saw every day as he arrived to work at the South door entrance of the Capitol building.

“He was a good officer, but also very personable,” said Ricci, who now serves as spokesman for Maryland Gov. Larry Hogan (R). “He was just what you’d want at that door with so many people coming through.”

Ricci recalled Sicknick’s sharp sense of humor. “He was funny,” he said. “Some officers, I recall, would be having political or sports debates . . . and he was a bit of a ball buster.”

Behringer, who was Pelosi’s communication adviser from 2016 to 2017 and is now a director of a Washington-based communications firm, said she bonded with Sicknick at a metal detector at an entrance near the Longworth House Office Building.

He poked fun at her, and she back. A word about Trump was met with a word about Pelosi, who at the time was the minority leader. While they both enjoyed the outdoors, she boated and ate crabs, and spent time with her parents who live on the Chesapeake Bay. He hunted and fished. He jabbed. “I gave it right back,” she said.

Each assured the other their candidate would become president. The day after the 2016 election, Behringer said she arrived at work despondent, and found officers holding the doors open wide for her. Sicknick was waiting, not to joke or to gloat, but to console.

“He held me as I cried,” Behringer recalled. “He knew how hard it was going to be for me. Neither of us said a thing. No words needed to be exchanged. I knew that even though it was a celebratory day for him, he was going to hold me up.”

Julie Tate, Ovetta Wiggins, Josh Dawsey and Spencer S. Hsu contributed to this report.