New details emerged about the twisted mind of alleged Florida school shooter Nikolas Cruz — including that at age 5, he watched his father die of a heart attack, and was bullied in later years by his younger brother.

He was also once seen clutching a dead bird to his crotch during reading class at Marjory Stoneman Douglas High School, in the school year before he allegedly turned it into his own personal slaughterhouse.

“He always stared at everyone,” senior Tyra Hemans told the Sun-Sentinel of sitting across from Cruz last year during her first-period reading class. They were both juniors then.

One day, she told the Sun-Sentinel, she saw that Cruz was holding his hand to his crotch.

“I looked close and I saw he was holding a dead bird near his genitalia,” Hemans told the paper.

“I saw some feathers and I knew it was a bird. That was disturbing,” she said.

“But I just looked away because it wasn’t my business.”

Hemans now wonders if Cruz had been carrying the dead bird around in his lunch box.

She said she never told a teacher, but did tell her best friend — Meadow Pollack, one of the 17 people whom Cruz allegedly murdered during his Feb. 14 rampage with an AR-15 assault weapon.

“We kept it to ourselves,” Hemans said of the bird.

Pollack and Hemans had tried to befriend the lonely outcast, Hemans said, lending him their cellphones during class so that he could use them for assignments.

A tipster later told the FBI that Cruz took a dead bird into the kitchen of his home and cut it open to see what was inside, the Sun-Sentinel reported.

The paper also reported that when Cruz was just 5, he came into his kitchen crying. When mom Lynda asked him, “What’s the matter, did Daddy punish you?,” he answered, “Nope. Daddy’s dead.”

After the Feb. 14 massacre, Cruz’s brother, Zachary, 18, admitted to Palm Beach County sheriff’s deputies that he and his friends had bullied Cruz — and said he ­regretted doing so.

“Zachary wishes that he had been ‘nicer’ to his brother,” the agency’s report said, according to the Sun-Sentinel.

Cruz had tried to shoot many more people at the high school, the Miami Herald reported — but was thwarted by the hurricane-resistant glass windows in a third-floor teachers lounge.

The glass cracked but didn’t break, preventing Cruz from firing his weapon at the terrified students fleeing for cover in the courtyard below, the paper said.