WHILE CHILDREN were learning basic math in the John Paul Jones Middle School in Kensington, Eugenio Santiago-Mejias was doing math of another sort: adding up the money he and two cohorts allegedly made from the illegal sale of guns in the neighborhood of the school.

But thanks to a yearlong undercover operation by members of the Bureau of Alcohol, Tobacco, Firearms and Explosives and local law enforcement, Santiago-Mejias and his cronies likely will be doing yet another mathematical exercise - tallying up the lengthy prison sentences they'll get if convicted of dealing in illegal firearms.

The arrests were announced at a news conference yesterday by U.S. Attorney Patrick L. Meehan outside the school, on Memphis Street near Cambria.

"We are here to announce the indictment of three Philadelphia men for illegally dealing in firearms right here on the city streets," Meehan said.

"Over the course of the last year, 22 firearms were trafficked in this particular neighborhood - eighteen of them within yards of this middle school."

Santiago-Mejias, 47, of Cambria Street near Tulip, is the accused leader, while Jose Luis Guzman, 23, and Jairo Correa-Merejildo, 21, both of Philadelphia, had major responsibilities in the ring, Meehan said.

Of Santiago-Mejias, Meehan said, "He was the go-to guy" for guns in the neighborhood.

Meehan said Santiago-Mejias had sold an undercover agent a Beretta handgun, a .25-caliber semiautomatic and a 12-gauge shotgun, for a total take of $1,050.

Thinking he had a good customer, Meehan said, a week later Santiago-Mejias sold more weapons to the agent: a fully loaded rifle, a SIG Arms M11 pistol, a 9-mm pistol and a loaded .380-caliber handgun. He collected $3,100 for those sales.

"These guys deliberately trafficked guns to the highest bidder," Meehan said, noting that his office isn't sure yet if any of the weapons sold had been used in previous crimes. Nevertheless, he said, it seemed certain that the three suspects contributed to the surge of handgun straw purchases in the neighborhood.

Many of the handguns were sold with their serial numbers filed off, making tracing that much harder, officials said.

According to ATF special agent Tom Stankiewicz, the trio would ply their trade at all hours, with little regard for the numerous children playing in the neighborhood and showing no concern for the safety of the children at Jones Middle School.

"They showed a complete lack of respect for the people in this neighborhood," said Stankiewicz. He said guns were also sold out of Papi's grocery store at Cambria and Memphis.

The owners of the store were apparently unaware the sales were going down in their establishment, and they have not been charged with any crime, Meehan said.

If convicted of all charges, Santiago-Mejias faces 30 years to life in prison, with a mandatory sentence of at least 15 years; Guzman and Correa-Merejildo each face jail sentences of up to 15 years.

Santiago-Mejias faces the stiffer penalty because he is a convicted felon who is not allowed to possess a gun.

"They were part of a continuing criminal enterprise," said Meehan, adding that the investigation is ongoing. "We want to trace each gun, and find the network putting them in the streets." *