Transgender Minneapolis school shooter Robin Westman targeted his former Catholic school’s first Mass of the year because he wanted to kill as many kids as possible, according to chilling, detailed notes he wrote.

Police also revealed that he tried to enter the school via the front door, but it had been barricaded, preventing a potentially even greater slaughter.

The killer said that he targeted Annunciation Catholic School and the church next door because he knew it well — his mother worked there as a secretary and he attended it as a child.

“I really want to shoot down the rows of pews at all the kids thinking they can hide lol. They are going to be trapped in those pews! Easy targets! Waiting ducks!” he wrote in the Aug. 7 entry of his twisted handwritten diary that he shared on YouTube before Wednesday’s mass shooting that killed two young kids and injured 18 others.

The killer made meticulous plans, attending church several times to ascertain entry points and potential obstacles — as well as picking a time when he knew the children would be there without their parents.

“The school schedule is publicly posted online so I should go through that and look for events that look promising. Oooo! Aug. 27th, that is an all-school mass!” the gunman put in the July 25 entry written in a homespun code of Russian Cyrillic and English words, which was translated by The Post.

“It starts at 8:15 so that means it’s after school starts and most parents will probably not be there!” the missive continued.

The entry for Aug. 8 reads, “I think my next best option is going through the south foyer doors that everyone uses. The church office opens at eight and there will be employees in the foyer, probably teachers too on that day. I want to be seen as little as possible before I start shooting.”

On Aug. 10, he wrote, “I bet the front doors will be unlocked today. I will go to Starbucks at 8:40, chill there, [then] walk to the church at 9:30, arrive about three minutes late,” describing his trial run of the shooting.

“I will walk down the west side and try door 22 on my way to the south doors. After Mass, I will try to exit out of the main doors and walk down the east side to go to the office,” he wrote

He said he picked the target because of his familiarity with it.

“I went to that school so I know the layout. I don’t think I really need to do that much recon at the start of the school,” he wrote.

Law enforcement said Westman planned his shooting to kill as many children as possible.

The sadistic killer “appeared to hate all of us,” said Acting US Attorney for Minnesota Joseph Thompson, as he described investigations into the motive.

“I won’t dignify the attacker’s words by repeating them, they are horrific and vile. But in short, the attacker wanted to watch children suffer,” he said at a press conference on Thursday.

“More than anything, the shooter wanted to kill children. The shooter was obsessed with the idea of killing children,” Thompson said, speaking alongside Minneapolis Police Chief Brian O’Hara.

Westman’s diary lays out how carefully he planned his attack on the Annunciation Catholic School and Church, even down to the car spot he would park in.

“The parking spot at Annunciation I wanted to use is a handicap spot, so it might be suspicious if I park there. It shouldn’t be a big deal initially, I don’t think I will be in my car long or even park in a spot now anyway,” he wrote.

He also looked for anyone who might be capable of challenging him.

“Both times I went to the church, I didn’t see any men under the age of 55. I only saw old people and some mom-aged ladies, some with very young kids,” he said.

“Even still, these boomers could be packing.”

Two kids — Fletcher Merkel, 8, and a 10-year-old student — “were killed where they sat” in pews when Westman opened fire through the church’s stained-glass windows. Another 17 people — including 14 kids — were injured, some critically.