WHEN POLICE phoned early yesterday, the mother of an 11-year-old boy assured them that her son was at home in bed.

But surveillance cameras at the Suburban Armory in Delaware County show he was miles from his Philadelphia home.

Her little boy, police said, had been arrested as part of a teenage posse that tried to pull off a brazen gun-shop heist in Collingdale Monday night by dismantling the external alarm system and throwing a boulder through the window.

They arrived on public transportation and used a pair of schoolbags to carry 14 semiautomatic handguns - plus the owner's loaded .38-caliber revolver - and about 250 rounds of ammunition stolen shortly before midnight, authorities said.

"I'm shocked an 11-year-old, first of all, was even out at that time, let alone breaking into a gun shop," said Joseph Galiano, a certified NRA instructor and owner of the Suburban Armory, on MacDade Boulevard.

Collingdale Police Chief Robert Adams said the two adults and three juveniles were smart enough to snip the phone lines behind the building, and that they may have tested police response time recently by tripping the alarm.

But their escape plan fell short. They didn't have a getaway car, and police quickly apprehended two of the suspects outside a Pat's Pizzeria around the corner from the Collingdale police station. Two others were busted at a SEPTA bus stop down the road.

The 11-year-old, Adams said, was arrested on the bus when the driver got caught up in a traffic jam that was caused by the police investigation into the crime.

"I wasn't stunned, I was shocked. I just could not believe that an 11-year-old kid would be involved in something like this. It's just deplorable," Adams said, adding that the boy was "directly involved" in the burglary.

"It's just a scary thought that in today's society kids are stealing guns like this and putting them back on the street," he said.

The two adults, Leroy Taylor, 19, of Philadelphia, and Khalik Keyser, 18, of Chester, were arraigned yesterday on charges of burglary, theft, criminal trespassing and related offenses. In 2005, Keyser pleaded guilty to attempted murder and weapons charges in Philadelphia and is on probation, according to court records.

Neither commented as they were led from the police station.

Besides the 11-year-old, police arrested Keyser's 16-year-old brother and a 15-year-old Collingdale boy. The Daily News is withholding their names because they are minors. They were awaiting a hearing yesterday in juvenile court.

"This is the scary underbelly of the activity that's going on that has resulted in a number of shots-fired incidents, aggravated assaults with firearms and, in fact, gunshot homicides not only here in Delaware County but in Philadelphia and throughout the region," said Delaware County District Attorney G. Michael Green.

"These kinds of weapons on the street are the type of weapons that we frequently find used in the commission of crimes - obviously violent crimes and homicides," Green said, adding that Chester recently experienced six homicides in a week.

Police have not determined what the thieves planned to do with the guns - two Glocks, a Ruger, four Smith & Wessons, two Cobras, a Bersa, a Springfield, two Tauruses, a Sky and a Beretta - but Green said it appeared from the quantity that they were going to be resold.The thieves might have gotten away if not for a witness who called 9-1-1 and identified all five suspects.

"It just goes to show you how important the community is," Adams said. "Without the witness last night, I do not feel these arrests would have been possible."

Galiano's shattered door was boarded up yesterday, but he plans to reopen the armory soon. "That's why I sell guns," he said. "So people can protect themselves from the bad element."

"Very shocking," said Collingdale Officer Patrick Crozier, who responded to the burglary. "I don't know what was going through his head." *

Staff writer Christine Olley contributed to this report.