Hattie Louise James was sitting on her front porch in Charlotte, N.C., when two police detectives emerged from their car. There had been a shooting, they said. Two officers were dead. The gun had been traced back to her.

“I liked to had another heart attack,” said the 72-year-old James, a retired hospital worker.

The .32-caliber revolver used to kill Charlotte-Mecklenburg police officers Sean Clark and Jeff Shelton in April 2007 started out as a legally owned weapon. James bought it in 1991 at Hyatt Coin and Gun Shop in Charlotte, but it was stolen a year later from her husband’s car. Fifteen years after that, it passed into the hands of 25-year-old Demeatrius Montgomery. This September, Montgomery was convicted of killing the officers outside a low-income housing complex in northeast Charlotte.

Clark and Shelton are two of 511 police officers killed by firearms in the United States from the beginning of 2000 through Sept. 30 of this year.

Until now, no one has conducted a comprehensive study of how the killers got their guns.

To trace these guns, The Washington Post did a yearlong investigation, including building a database of every police officer shot to death in the past decade. (More than 1,900 officers were wounded by firearms during the same period.) Through documents and interviews, The Post was able to track how the suspects obtained their weapons in 341 of the deaths.

This kind of analysis is made more difficult by a law passed by Congress in 2003 that bars federal law enforcement from releasing information that links guns used in crimes back to the original purchasers. To penetrate that secrecy, The Post interviewed more than 350 police officials, prosecutors, defense attorneys, judges, gun dealers, gun buyers, suspects and survivors. In 30 cases, the newspaper obtained confidential firearms traces generated by the Bureau of Alcohol, Tobacco, Firearms and Explosives. The ATF’s reports track guns recovered at crime scenes back to dealers and original buyers, listing a gun’s model, caliber and serial number.

The Post review shows how guns got into the hands of police officers’ killers and — in a nation with more than 250 million guns in circulation — how a moment of panic can have deadly consequences.

Among the findings:

nLegal purchase was the leading source of weapons used to kill police officers. In 107 slayings, the killers acquired their firearms legally. In 170 deaths, The Post could not determine how the shooters got their guns, including 29 killings in which weapons were not recovered.

nStolen guns turned up in 77 deaths. Separately, guns obtained or taken from relatives or friends who legally owned them were used in 46 killings. Fifty-one officers were killed when their department-issued firearms or other officers’ guns were turned against them. In 41 instances, guns were illegally obtained on the streets through sale or barter. Sixteen times, someone bought a weapon for a person prohibited from having a gun, an unlawful transaction known as a straw purchase. The straw buyers were federally prosecuted in fewer than half of those cases. Three were illegally purchased at gun shows or from private sellers.

nThe two deadliest situations for police are traffic stops and domestic disputes. Ninety-one of the officers were killed while making traffic stops; 76 were responding to domestic disturbance calls. The officers killed at traffic stops were generally slain by felons wielding illegal guns; the weapons used to kill police in domestic situations were often obtained through legal purchases. Only 13 percent of the weapons in the traffic stops were legal, compared with 47 percent in the domestic calls.

nMore than 200 of the shooters were felons who were prohibited by federal law from possessing firearms. Many had spent time in prison for illegal handgun possession. At least 45 were on probation or parole when they killed an officer. At least four were previously convicted of murder or manslaughter, including a Texas man who had done time for two separate slayings and was on parole at the time he killed his third victim: a 40-year-old sheriff’s deputy with a wife and three children.

nHandguns were used to kill 365 officers; long guns — rifles and shotguns — were used to kill 140 officers. (Two were killed with a rifle and a handgun, and in four cases, The Post could not determine the type of weapon.) The ratio of handguns to long guns in The Post review — about 70 percent to 30 percent — is close to being the inverse of the ratio of all guns in the nation: 40 percent handguns to 60 percent long guns. But the ratio found by The Post matches that for U.S. homicides in general, experts say, reflecting the preference among criminals for handguns because they are generally cheaper and easier to conceal. The most common handgun used was the 9 mm semiautomatic pistol, which was used to kill at least 85 of the police officers.

nWith a median age of 27, the shooters were generally younger than the population at large, while the officers’ median age of 36 matched the country’s. Forty-two of the killers were 18 or younger, including four 15-year-olds. The oldest shooter was 77. At least six of the suspects had been released early from prison sentences for previous crimes, including a man who was freed a day before gunning down an officer.

Among the officers killed were a newly minted officer fresh from the police academy; a 31-year veteran two weeks from retirement; and one slain moments after having dinner with his family on Christmas Eve 2000. The youngest slain officer was 19; the oldest was 76.

nTo some extent, the geographic distribution of the killings tracks population size and the violent-crime rate. The two most populous states led the nation in police officer shooting deaths: California with 47 and Texas with 46. Next were Louisiana with 28 and Florida with 27, even though Florida has four times as many residents. Louisiana has the nation’s highest rate of police killings per capita and the nation’s highest overall rate of death by gunfire, according to a study by the Violence Policy Center, a nonprofit group that advocates gun control.

One notable exception to the population trend appears to be New York, which has the third-largest number of residents but is tied for 13th in police killings with 16. New York is known for having some of the toughest gun laws in the country.

In general, states with looser gun laws had higher rates of fatal shootings of police officers, overall handgun killings, and sales of weapons that were used in crimes in other states, according to a 2008 study underwritten by Mayors Against Illegal Guns, a coalition of 300 mayors led by New York’s Michael Bloomberg. That study looked at police shootings in the aggregate but did not trace the origin of the guns.

The 511 police officers in The Post study are among more than 95,000 Americans killed by people using firearms in the past decade.

“It is extremely easy in this country for anyone who wants to get a weapon to obtain one, particularly a handgun,” said Norfolk, Va., Police Chief Bruce P. Marquis, whose department has lost five officers to guns since 2001. “There is not a lot we can do about it unless the laws are changed to restrict guns to make it harder to get them or severely punish those who knowingly obtain weapons stolen or used in other crimes.”

Federal law prohibits felons, people who have been committed to an institution for mental illness, and drug users from buying a gun. Buyers of handguns must be at least 21 years old, and the minimum age for buying a rifle or shotgun is 18. States have wide latitude to set limits on how many handguns may be bought at a time and to require additional background checks, purchase permits and the reporting of lost or stolen guns.

“There’s such a disparity between the gun laws in different states,” said Lt. Howard Schechter, head of the forensic investigation unit for Albany, N.Y., police. “Down South, their feelings about guns and gun control are completely different. Both Carolinas, Florida, Georgia, they’re generally very easy places to get guns.”

The number of legally owned firearms among the guns The Post was able to track — 107 out of 341 deaths — surprised Garen Wintemute, a professor of emergency medicine and director of the Violence Prevention Research Program at the University of California at Davis.

“That’s high,” Wintemute said. “That’s very unusual.”

Wintemute said people charged with felonies often plead guilty to misdemeanors. And he noted that although a felony conviction makes it a federal crime to possess a gun, a misdemeanor carries no such restriction.

“We are finding here cases in which felons have been able to acquire guns even though they shouldn’t, but we are also finding cases in which people who have criminal misdemeanor records but remain eligible to buy guns do buy those guns and then kill cops with them,” he said. “Any effort to find a pattern in these tragedies is helpful, because patterns often lead to solutions.”

In the case of Hattie James’ gun in North Carolina, a stolen weapon went missing for more than a decade before it surfaced in the killing of two officers.

Larry Hyatt is the owner of Hyatt Coin, the Charlotte, N.C., store that sold the gun to James originally. He said he had heard rumors that a gun from his store had been used to kill the officers, but he was not certain of it until called by a Post reporter.

“That was so horrible what happened,” he said. “It just makes me sick to think about it. Do I feel bad? You doggone right I feel bad.”

Hyatt, 63, said he runs an honest business, family-owned since 1959. His 81-year-old mother still runs the cash register and occasionally lectures buyers on the need to be legitimate. “We do everything we can and double- and triple-check to try and do everything right,” he said.

Hyatt said it is difficult to keep guns out of the hands of criminals, because once a firearm leaves the store it can be stolen or passed around.

“What it is is hundreds of thousands of random events — thefts, deaths — outside the federally licensed, controlled system, guns that are being stolen, sold hand to hand and inherited,” he said. “That’s why it’s so difficult to get a handle on it. It is a problem, but it’s not with us.”