What caused the Celtics to squander a 20-point second-half lead and lose to the New York Knicks — then suffer the exact same ignominy again two nights later?

There isn’t one all-encompassing answer to that question. Defensive breakdowns played a part. So did questionable late-game coaching decisions, like Joe Mazzulla pushing New York into the bonus with intentional fouls and keeping his final timeout holstered in crunch time. Player availability, too, with Kristaps Porzingis limited by a long-running illness and Sam Hauser sidelined with an ankle sprain.

But the simplest, most pertinent and most alarming explanation is this:

The team that just set the NBA record for made 3-pointers in a season suddenly cannot shoot.

Games 1 and 2 of the Eastern Conference semifinals at TD Garden featured, without hyperbole, Boston’s two worst shooting performances of the season.

The Celtics did not shoot worse than 36.5% in any of their 82 regular-season games or five first-round playoff games. On Monday night against the Knicks, they shot 35.1% and lost 108-105 in overtime. On Wednesday, it was 36.2% in a 91-90 defeat.

In both games, the Celtics led by 20 points after the halfway mark of the third quarter. From those points forward, they went a combined 14-for-67 from the field (20.9%) and 8-for-42 from 3-point range (19.0%).

Overall, Boston has taken 100 threes in its second-round series and missed a staggering 75 of them. And its top players have been the worst offenders.

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Jayson Tatum, after torching Orlando’s vaunted defense in the prior round, was 7-for-23 in Game 1 and 5-for-19 in Game 2. He’s attempted 20 threes and made five.

Jaylen Brown has posted similar stat lines: 7-for-20 in Game 1, 8-for-23 in Game 2, 3-for-17 from deep between the two, with just three points after halftime Wednesday night.

Derrick White, the owner of the Celtics’ single-season 3-point record, made his first two triples in Game 2, then bricked eight of his final nine.

“We just had the ability to make plays tonight, and we didn’t,” Brown said after Wednesday’s loss. “Me, Jayson, Derrick — we got a lot of great looks, as we’ve just got to convert. We’ve got to make plays. We just didn’t make plays tonight, and that was the story of the game. Our defense did enough. We got some great open looks. We’ve got to convert those.”

After a Payton Pritchard 3-pointer put the Celtics ahead 84-68 with 8:40 remaining in Game 2, they proceeded to miss their next 13 shots — four by Brown, three by White, two by Tatum, two by Pritchard and two by Al Horford, who started over Porzingis and went 2-for-11 (0-for-5 from three). Boston went more than eight consecutive minutes without a made field goal, during which New York completed its comeback, pulling ahead on an 18-footer by Jalen Brunson with 1:59 remaining.

Tatum finally ended that drought with a go-ahead end-to-end dunk with 18.1 seconds to play, but Brunson free throws on the ensuing possession provided the winning margin for New York. Tatum was unable to get a shot off against Mikal Bridges as time expired, the same fate that befell Brown in the waning moments of Game 1.

(Tatum did not speak with reporters postgame because the arena was evacuated before his news conference could begin. The cause: a minor fire in a TD Garden elevator shaft, a fitting end to a dysfunctional night for the building’s tenants.)

The Knicks boast three talented wing defenders in Bridges, OG Anunoby and Josh Hart, but they had minimal success against Tatum, Brown and Co. during the regular season. In four meetings, the Celtics shot 50% from the field and 43.5% from three while averaging 125.0 points per game — marks they haven’t sniffed in this series.

“All throughout the game, we could’ve probably shot the ball better, did this better, then we (could have pulled) away even more,” said Porzingis, whose illness limited him to three fourth-quarter minutes over the two games. “But they hung around, and similar result. It sucks really bad right now. But we got a lot of basketball to be played, and let’s see where this goes.”

Wednesday’s loss pushed the Celtics into a jam they’ve successfully escaped just twice in franchise history. The other 16 times they fell behind 0-2 in a best-of-seven series, they lost, as such teams across the NBA have done 85.7% of the team. The lone exceptions for Boston came in the opening round of the 2017 playoffs, when the Celtics won four straight to knock off the eighth-seeded Chicago Bulls in six, and the 1969 NBA Finals, when Bill Russell won his final title in a seven-game epic against Wilt Chamberlain’s Lakers.

These Celtics can draw optimism from the fact that they were the league’s best road team this season (33-8 away from Boston) and among its most resilient, having not lost three straight games in nearly two full years (since the 2023 East finals against Miami). Also, the results of their two monumentally poor shooting performances were a three-point loss in overtime and a one-point loss, both of which they had a chance to tie or win on the final possession.

But those bafflingly inefficient outings have left the Celtics with little room for error. After becoming the first team in the play-by-play era (since 1996-97) to lose multiple games after leading by 20-plus in the same postseason (never mind the same series), they now must win four of their next five to avoid becoming the sixth straight defending NBA champion to bow out before the conference finals.

“Not an ideal situation, being down 0-2,” said Brown, who called the series deficit “inexcusable.” “But what’s done is done. We’ve got to make sure we’re ready to come out … for Game 3. I think we’re due for a lot of makes. I think we had a lot of great shots tonight that, once again, didn’t go in. We’ve just got to relax, take a deep breath, come out and play Celtic basketball. Just like we’re down 0-2, we can tie this thing back up.”