Of all the people who watched the riot at the U.S. Capitol unfold last week, no one saw it the way a great-grandpa sitting on his couch in Chicago did.

Terrance W. Gainer, 73, was once the chief of the U.S. Capitol Police and the Senate’s sergeant at arms. He used to be the one in charge of preventing the very calamity at the Capitol he saw on his television and computer screens, a calamity he used to believe could never happen.

And not only was it happening, it was being done by the same folks who proudly proclaim they support the blue.

Few images capture the pure feeling of betrayal and disbelief he had more than the photos and videos of police officers being pummeled, jostled, dragged, stabbed, sprayed, punched and trampled by the mob that rioted in our nation’s Capitol on Jan. 6.

“For the life of me, the majority of them seem to be pro-Trump. And I didn’t get that,” Gainer said. “The same way I didn’t get how the anarchists who were coming there claiming to be Trumpettes and love the police then fought the police.”

Gainer’s is a huge law enforcement and military family that sprawls across the Army, Navy, FBI and police. One of his sons is a D.C. police sergeant who was on duty that day.

“They’re not on our side anymore!” I heard someone scream as I got caught in a surge of people shoving through the open space made when a bicycle fence — one of those low-level barricades you usually see at parades — was knocked over on the west side of the Capitol. “[Expletive] the police!”

“Storm the Capitol!” said a guy wearing a Trump flag as a cape.

“Don’t let the police through, they’re drones for a Democratic mayor!” screamed another as he shoved his way deeper into the crowd surging up the Capitol’s steps.

This was the “Blue Lives Matter” crowd, the same folks who hoisted those black and gray flags with a bold blue stripe as a defiant response to the summer’s Black Lives Matter demonstrations.

Guess it was only for show. And that’s something that baffled and sickened Gainer.

He’s seen it all. The guy got his start in Chicago, at the 1968 Democratic National Convention. He served in the Vietnam War and did some time with Illinois state policebefore coming to D.C. as an assistant chief. He became Capitol Police chief in 2002, stayed until 2006, then was the Senate sergeant at arms until he retired in 2014.

He always answered my calls when I was a night police reporter in D.C., and once helped pull me out of a violent crowd and cloud of tear gas when I was covering the IMF/World Bank protests near Dupont Circle in 2000.

So, 21 years and another violent crowd later, I thought of Gainer and wondered what he was thinking about the astonishing scene unfolding in his once home turf.

“I was sitting home watching it,” he told me, once we finally talked this week. “And I did turn to my wife and say: ‘Wow. We only have bicycle fence up, and I just don’t see a lot of officers.”

“When they started making their move up the hill, I was then getting calls from folks out there,” he said from his home in Chicago, where he took calls from old friends and colleagues in D.C. “And they said, ‘You think they could breach the Capitol?’ And I said, ‘No way. No. Absolutely not.’ ”

But then they did.

And then Gainer was on the phone the rest of the day, watching and talking about the scenario he believed would never happen.

He’s angry about it, and devastated for the officers and what they endured and who they lost. Officer Brian D. Sicknick, 42, died after he was injured in the mob. Officer Howard Liebengood, 51, died by suicide after the riot. Gainer knew Liebengood; the two used to talk about the officer’s time as a racecar driver.

“They were really frazzled,” he said, after speaking with some officers after the riot. “The death of Brian, the suicide of Howie, I was genuinely concerned for their mental well-being.”

And Gainer worried that the other officers there may also be struggling, feeling like they somehow failed when America watched the Capitol surrounded and invaded. Or feeling betrayed by fellow officers, some of whom were suspended for being friendly with the invaders or are being investigated for helping them.

“You just have to support and support and support them,” he said. “We have to reinforce with them that they didn’t lose.”

“They regained the interior, kept the members safe from all the marauders and kept the staff safe from all the marauders, and actually helped reconstitute the constitutional process,” he said. “They were successful, given the tools they had and the numbers they had, and they just have to believe that.”

The chief in charge when all this went down, Steven Sund, stepped down, after taking plenty of fire for the way the riot was anticipated and handled.

Sund later told The Washington Post that well before Jan. 6, he asked for permission to get the National Guard on standby. But he said House Sergeant at Arms Paul Irving, whose authority overrules the Capitol Police, told him the “optics” of formally declaring an emergency ahead of the demonstration weren’t good and denied his request.

Gainer said the dynamic is problematic and expects congressional leadership will be answering — for months, and maybe for years — for the decisions made that day, the choice to prepare for thousands of angry rioters without the help of military support or even a full staff.

Despite the untenable odds and nightmare attacks, there were moments of real patriotism. “You’ll be hearing those stories,” Gainer said.

Like Officer Eugene Goodman, the Black war veteran who lured a mob of angry White rioters away from senators in a move captured on video.

Or the dozens of officers who were beaten, dragged, sprayed and pummeled, but kept working.

Gainer watched the television coverage, recognizing faces of the ones he knew and trained, horrified at what was happening, proud when he saw them stand their ground and fight back.

“I hear it often that [Capitol Police] aren’t like other police departments because they don’t have a community, and I say, ‘Boy, do you have it wrong,’ ” he said. “They’re America’s police department.”

Twitter: @petulad

Read more Petula Dvorak: