A 13-year-old New Jersey boy was killed and another seriously hurt by a huge bullmastiff that escaped its Paterson home and mauled the children as they were walking home from school, authorities said.

The kids ran in different directions and got separated during the bloody Friday 3 p.m. attack at Crosby and Rossiter avenues, Paterson police said. But only one made it to safety.

Hours later, the child returned to the scene with three adults to try to find his best friend, who had run toward a creek during the frantic chase.

“The boy said, ‘I got bit by a dog and I can’t find my friend, we’re looking for him,’” said neighbor Maria Zachus, 49.

“Then I heard the guy say, ‘He’s here, he’s here! I found him!’”

One of the adults was the missing boy’s dad.

“The father was screaming and crying, ‘My son, my son, my son — but why here, oh God why, why?’”

The grim discovery was made at 11:30 p.m., near Rossiter Avenue and Hill Street. The boy was declared dead soon after, according to authorities. An autopsy has been scheduled.

Carmen Baez, 66, was walking with her husband when she encountered the bleeding, panicked boy minutes after the vicious attack.

“I thought that maybe he was in a fight or something,” she told The Post. “I said, ‘Hey what happened to you?’ he said, ‘a dog bit me.’ … I saw a hole in the palm of his hand. I saw the blood running down it.”

The boy would accept only a paper towel to wrap around his bloody wound, and refused Baez’s offer to contact his mom, saying, “My mother’s working,” said Baez.

The injured child ran off, without mentioning his friend.

“If you’d said, ‘I have a friend that is also hurt,’ I would’ve called the cops, but he never even mentioned that,” Baez said.

The surviving teen was treated at St. Joseph’s Medical Center for a bite to his arm, family lawyer Adam Rothenberg said. It’s unclear who transported the injured child to the hospital.

The bloodshed only ended when the owner of the 115-pound dog, identified by four neighbors as Paul Clarke, of Sherwood Avenue, stabbed the animal several times, Paterson Animal Control Officer John DeCando told nj.com.

The hulking animal, named Trigger, was “humanely euthanized”by animal control officers. The remains will be tested for disease, authorities said.

DeCando described the attack as “the worst that I’ve seen” in 37 years on the job.

Trigger could be intimidating, said Baez and other neighbors.

Dog owner’s wife, Sharon Clarke, insisted that Trigger was “friendly,” but conceded the animal had once bitten her son’s friend.

If not for her husband pulling a knife, “He’d be killed,” she added.

The dog bit a family friend last year but the attack was not reported, residents told The Post.

Baez said the animal once charged her while she walked by its Sherwood Avenue home, but the animal was stopped by the property’s fence.

“It was scary,” she said.

“It’s a horrendous situation,” Rothenberg told The Post. “My client fled for his life.”