Outside a baby shower in Landover three years ago, Erik Kenneth Dixon snapped. As he argued with his sister and her boyfriend in a parking lot, the 25-year-old man whipped out a .45-caliber Glock and shot her in the leg. Then he chased down her boyfriend, firing between cars and at the running man's feet until he slipped on wet grass. As the prone man held his hands up in futile defense, Dixon executed him, firing seven times.

By law, Dixon was prohibited from owning a gun. He had spent almost three years in prison for shooting at a man. But three months before the baby-shower killing, he gave his girlfriend $335 and took her to an old brick house on a commercial strip just beyond the District line in Forestville, home to a gun shop called Realco.

"He knew which one he wanted and picked it out," the woman would later tell police.

Dixon's Glock was one of 86 guns sold by Realco that have been linked to homicide cases during the past 18 years, far outstripping the total from any other store in the region, a Washington Post investigation has found. Over that period, police have recovered more than 2,500 guns sold by the shop, including over 300 used in non-fatal shootings, assaults and robberies.

Realco has been known as a leading seller of "crime guns" seized by local police, but a year-long Post investigation reveals the magnitude of Realco's pattern and links the guns sold by the store to specific crimes. The Post compiled its own databases of more than 35,000 gun traces by mining unpublicized state databases and local police evidence logs.

The Post investigation found that a small percentage of gun stores sells most of the weapons recovered by police in crimes - re-confirming the major finding of studies that came out before federal gun-tracing data were removed from public view by an act of Congress in 2003. For the most part, these sales are legal, but an unknown number involve persons who buy for those who cannot, including convicted felons such as Dixon, in a process known as a "straw purchase." Such sales are illegal for the buyer and the store, if it knowingly allows a straw purchase. But cases are hard to prove. Law enforcement officials rarely prosecute gun stores, deterred by high bureaucratic hurdles, political pressure and laws that make convictions difficult.

The investigation also found that:

l Nearly two out of three guns sold in Virginia since 1998 and recovered by local authorities came from about 1 percent of the state's dealers - 40 out of the 3,400 selling guns. Most of those 40 had received government warnings that their licenses were in jeopardy because of regulatory violations. But only four had their licenses revoked, and all are still legally selling guns after transferring their licenses, reapplying or re-licensing under new owners.

l A gun store in Portsmouth, Va., transformed over the past seven years from a modest family-owned business into one of the state's top sellers of "crime guns," leading Virginia in the category of how quickly its guns moved from the sales counter to crime scenes.

l The federal Bureau of Alcohol, Tobacco, Firearms and Explosives, which investigates gun trafficking and regulates the firearms industry, is hamstrung by the law, politics and bureaucracy. The agency still has the same number of agents it had three decades ago. It can take as long as eight years between inspections of gun stores. And even when inspectors turn up evidence of missing guns, they cannot compel a dealer to take inventory.

In Maryland, Realco towers over the other 350 handgun dealers in the state as a source of guns confiscated in the District and Prince George's County, the most violent jurisdictions in the area. Nearly one out of three guns The Post traced to Maryland dealers came from Realco. The rest were spread among other shops across the state.

The store is a paradox for law enforcement and politicians. Its owners say they scrupulously follow handgun laws. State and federal regulators have documented only minor problems in numerous inspections.