ENGLEWOOD, Colo. — The Broncos were sitting pretty up 20-0 at halftime. Then the unthinkable happened. Literally.

In the last five years, teams who had a 20-plus point lead at halftime were a glistening 99-0. The Broncos were on their way to making that a perfect 100-0.

Then, the wheels fell off. In every aspect.

Despite dropping 20 points in the first half, Denver could only manage three points in the second half. On the other side of the ball, after pitching a perfect shutout in the first half, Vic Fangio’s defense couldn’t stop a fly in the second half, giving up touchdowns on all four possessions.

Questions were abound as the lead slowly and painfully drifted out of the Broncos’ hands. Answers came after the game.

The downfall began just before halftime—with 52 seconds left in the first half, to be exact.

After recovering the Vikings fumble on the kickoff, the Broncos had a chance to put the game away. Instead, Brandon Allen changed the tides of the game by immediately throwing an interception.

“Me and Noah [Fant] just weren’t on the same page of what we thought we were going to do,” Allen said, explaining why Fant seemed to stop on his route. “Probably a bad throw by me, really just trying to force it and a get a big play. I think that was on me for forcing it. Probably should have checked that one down.”

Fangio said in hindsight he could have ran the ball three times and kicked field goal as the half expired, but “you would have roasted me if I did that,” he said.

That momentum rolled straight into the second half when the Vikings couldn’t be held out of the end zone, including the back-breaking 54-yard touchdown strike from Kirk Cousins to Stefon Diggs to bring the Vikings within three points.

Initially, it appeared Chris Harris Jr. was flat-out burned on the big play by Diggs. However, Chris was without major help over the top.

“There was supposed to be a safety there, he got out of position and when you do that against this type of team, when they get it rolling like that, that will kill you,” Fangio said, putting the blame on the safeties. “And it killed us there.”

Kareem Jackson shouldered the blame after the game for being out of possession.

But despite blowing a 20-point lead, the Broncos had a chance. Driving into Vikings’ territory with less than five minutes left, only down four, Denver had an opportunity to put the forgettable second half in dark closet and never talk about it again.

But more questionable decisions followed.

Facing a 4th-and-1 on Minnesota’s 15-yard line, the Broncos let the clock run down over 20 seconds. As they were set at the line, ready to call a play, they instead called their final timeout.

“He couldn’t hear the call being made into his head and I could tell there was confusion and with it being fourth down, that being our last chance, I just wanted to call timeout and make sure we’re all on the same page,” Fangio said, explaining why the team burned their last timeout.

“It was loud,” Allen said. “We’re trying to get the play call into the headset and we’re having trouble hearing it. Trying to work it out.”

The timeout worked, setting up Denver in prime position to take the game as they sat at the four-yard line with 10 seconds left down four.

The Broncos had three plays to move the ball four points and win the game.

None of the plays went to Denver’s No. 1 offensive weapon, Courtland Sutton, who, by the way, had five catches for 113 yards and looked like an unstoppable force.

Allen said the reason Denver’s best receiver wasn’t targeted was that he was double-teamed on every play.

“Really shouldn’t go to him, you don’t want to throw it up in double coverage to him,” Allen explained. “I thought other guys separated, I was just trying to give us a chance, get the ball in the end zone. It didn’t work out.”

It didn’t work out is right. But now it’s a little more clear as to why it didn’t work out.