SAN ANTONIO — A methamphetamine dealer who gunned down a deputy during a traffic stop in Southern California. A man in Arizona who killed his ex-girlfriend’s parents and brother and snatched his children. A man who suffocated his baby daughter and left her body in a toolbag on an expressway overpass near Chicago.

Ordinarily, these would be death penalty cases. But these men fled to Mexico, thereby escaping the possibility of execution.

The reason: Mexico won’t send anyone back to America unless America gives assurances it won’t seek the death penalty — a 30-year-old policy that rankles some American prosecutors and enrages victims’ families.

“We find it extremely disturbing that the Mexican government would dictate to us, in Arizona, how we would enforce our laws at the same time they are complaining about our immigration laws,” said Barnett Lotstein, special assistant to the prosecutor in Maricopa County, Ariz., which includes Phoenix.

“Even in the most egregious cases, the Mexican authorities say, ‘No way,’ and that’s not justice. That’s an interference of Mexican authorities in our judicial process in Arizona.”

It may be about to happen again: A Marine accused of murdering a pregnant comrade in North Carolina and burning her remains in his backyard is believed to have fled to Mexico. Prosecutors said they have not decided whether to seek the death penalty. But if the Marine is captured in Mexico, capital punishment will be off the table.

Mexico routinely returns fugitives to America to face justice. But because of a 1978 treaty with America, Mexico, which has no death penalty, will not extradite anyone facing possible execution. To get their hands on a fugitive, American prosecutors must agree to seek no more than life in prison.

Other countries, including France and Canada, also demand such “death assurances.” But the problem is more common with Mexico, since it is often a quick drive from the crime scene for a large portion of America.

The Justice Department said death assurances from foreign countries are fairly common, but it had no immediate numbers. State Department officials said Mexico extradited 83 suspects to America in 2006. Most were wanted on drug or murder charges.

The American government typically pays more attention to those entering the country from Mexico than it does to those trying to leave America. But Texas authorities have begun making checks of vehicles and drivers heading south on the 25 international bridges that connect the state to Mexico. The initiative, announced in October, was originally intended to catch drug smugglers taking cash or stolen cars into Mexico, but “we would hope it would be a deterrent for fugitives” as well, a spokeswoman for Governor Rick Perry, Allison Castle, said.

In the North Carolina case, local authorities and the FBI are working with Mexican law enforcement to hunt down Corporal Cesar Laurean, a 21-year-old naturalized American citizen born in Mexico. He is accused of killing 20-year-old Lance Corporal Maria Lauterbach in mid-December, months after she accused him of rape.

Wanted posters and information on Mr. Laurean have been distributed to the Mexican press. Also recently, prosecutors in Dallas pledged not to seek the death penalty if Mexico extradites Ernesto Reyes, a man accused of killing and burning the body of a University of North Texas student last year. That extradition request is still pending.

John Walsh, host of TV’s long-running “America’s Most Wanted,” which plans to devote Saturday’s episode to the Marine case, said the delays and death-penalty compromises needed to get fugitives returned can be heartbreaking for victims’ families.

“It’s not about revenge. It’s not so much about closure. It’s about justice,” he said.

Mr. Lotstein, the prosecutor’s assistant in Phoenix, said the county has agreed to drop the death penalty in a number of cases: “The option we have is absolutely no justice, or partial justice.”