Maryland State Police said yesterday that they have opened a criminal investigation into Realco Guns Inc., a Prince George's County gun dealer that is the Washington region's biggest source of firearms that are used in crimes.

Investigators said they are trying to determine whether Realco knowingly has sold guns to "straw purchasers," people with clean legal records who buy guns on behalf of criminals and others who are prohibited from owning them.

State police said their probe is focusing on how quickly guns end up in the hands of criminals after they are sold by Realco. Officials would not discuss other aspects of their investigation, which they said has been underway for several months.

"We're definitely taking an aggressive look into Realco's business dealings," said Lt. Jack Simpson, an investigator with the state police's crime-gun enforcement unit. "We're looking into anything and everything."

"We have an awful lot of work to do," Simpson added. "This will probably be one of the longer and more in-depth cases we look at just because of the sheer number of traces."

The Washington Post reported in June that Realco was the source of 493 guns that were used in crimes from 1996 through 1998, more than double the number of such guns traced to any other dealer in the region, according to data from the U.S. Bureau of Alcohol, Tobacco and Firearms.

ATF tracing records show that about three-quarters of crime guns that were traced to Realco in 1997 had been sold by the store within the previous three years. Researchers believe that such a fast turnaround from legal purchase to use in a crime is an indicator of potential firearms trafficking.

"That's what we're going to take a look at--where the guns changed hands on the street," Simpson said. "We take a look at the secondary sales on the street and then backtrack and do interviews and see if anybody has anything to say."

The ATF data showed that nearly half the guns traced to Realco were used to commit crimes out of state. Realco operates out of a tiny shop on Marlboro Pike in Forestville, less than three miles from the District, where the sale of handguns is banned.

Realco's owner and president, Greg del Real, of Rockville, said yesterday that he was unaware of the investigation. He said he and his employees had broken no laws and complained that state police were unfairly targeting Realco.

"It's a pretext to put our store out of business," del Real said. "I wish Lieutenant Jack Simpson would get a life and go after all the gun shops in the state of Maryland instead of just picking on one in Prince George's County."

Del Real noted that Realco has not been prosecuted or cited for any violations during its more than 25 years in business--something that state, local and federal officials confirmed.

Mike Campbell, a spokesman for the ATF's Baltimore field office, said the federal firearms agency conducted a routine audit of Realco's sales records in May and found no serious problems.

"There have been no discrepancies or record-keeping problems that would even require a warning letter," Campbell said.

But he said the ATF audit only checked Realco's books and was not a criminal investigation.

Under Maryland law, any gun dealer who "knowingly and willfully" sells a weapon to a straw purchaser can be fined up to $25,000 and receive 10 years in prison for each violation. Authorities acknowledged, however, that it is difficult to prove that dealers "knowingly" make such sales.

Del Real said his employees are strict about screening customers and are quick to spot telltale signs of straw purchasers, such as someone with a criminal record who walks into a shop with his girlfriend and tells her which gun to buy.

"We're going out of our way to spot these people," he said. "But they're very smart. They get turned down here and go to another gun shop and try the same thing, or vice versa. They know how to play the game."

Meanwhile, Realco is feeling pressure from other corners.

Interfaith Action Communities, a coalition of religious groups and other community activists, recently sent a letter to del Real expressing alarm at the number of crimes being committed with guns sold by his store. The group asked to meet with del Real, but he has so far declined.

"It's just not acceptable to us, the number and percentage of his guns that fall into the hands of criminals," said the Rev. George Golden, of Mount Calvary Catholic Church in Forestville, which is just a few blocks east of Realco. "We do intend to hold him accountable."