LAWRENCE — Three 13-year-old boys were making plastic bottle bombs for at least two months using a recipe they found on the Internet site YouTube, until one of them was injured, police said.

The boys, all students at Oliver School, made so many of them that they can't remember if separate explosions last month on Jackson and Bruce streets were their doing, police said.

Their dangerous enterprise was uncovered yesterday after school officials asked one of the boys about facial injuries he had.

The boy said one of the homemade devices spewed sulfuric acid on his face last Friday and that he was treated at Lawrence General Hospital.

His injuries did not appear to be serious, fire Capt. William Lannon said.

"But if it went into his eyes he'd definitely have had some long-term serious problems," Lannon said.

The boy was holding the bottle at arm's length when it exploded.

That boy admitted to making the bomb so he could throw it at a girl's house, police Chief John Romero said. He was arrested yesterday afternoon. Police are seeking arrest warrants for the other two boys today. All three will be charged with illegal possession of an explosive item.

Ingredients include two common household cleaners combined in a bottle. The boys learned how to tailor the recipe for bigger explosions and they combined the ingredients in thick, plastic Gatorade bottles, which amplify detonation, police said.

The ingredients combine to create intense, high pressure, which causes the bottle to explode, Lannon said.

The teens were making the bombs so long they forgot where they've blown them up, Romero said.

"It's no joke," Romero said. "They've done so many in the area they don't know where they all were."

Last Friday, after the boy was burned, his friends took him to the YMCA at 40 Lawrence St., where they washed his face. When he still complained of pain they took him home and his family took him to the hospital.

After that bottle bomb exploded, the boys threw another one — a 16-ounce soft drink bottle — onto a flat roof at the rear of 82 Maple St.

Authorities yesterday found the bottle on the ground nearby, where they said they believe the wind had blown it from the roof.

Lannon said the bottle bombs most often make a big noise when they explode, but when placed in a confined space, such as a mailbox, they can blow up the mailbox.

"That's usually the application we find them in," Lannon said.

He said conditions would have to be unique for a bottle bomb to actually start a fire, most likely involving whatever the bomb landed on before exploding.

The bombs do have enough explosive force to blow out windows, he said.

The danger from the bombs is mostly related to the energy they release when they explode, and the chemical reaction, he said.

Romero said police asked the trio about two bottle bomb incidents that occurred on Feb. 20 at Jackson and Lawrence streets and at 245 Bruce St., but that the boys could not remember if they were involved.

"I don't think they were trying to deceive us. They've done so many they can't say whether they did or didn't do the Feb. 20 bottle bombs," Romero said.

Romero said the boys had been purchasing the household supplies they use to make the bombs from local stores, and that police would reach out to the stores to tell them what was being done with the products, in hopes that store owners would police themselves and not sell the ingredients to youths unless they were with a parent.

Staff reporter Jill Harmacinski contributed to this story.