Of the 10,000 or so impostors who have been caught lying about being Navy SEALs, James Edward Nalls may be the most notorious.

An Allegheny County jury this week convicted him of third-degree murder in the shooting of his 19-year-old girlfriend.

Mr. Nalls, 25, of West Mifflin, was the first fake SEAL to be convicted of so serious a crime, said Tucker Campion, a retired SEAL and a leader in a national organization that exposes those who falsely claim to have been members of the Navy's elite Sea, Air and Land forces.

"We've seen phony SEALs who became kidnappers and abusers, but this guy is first to kill somebody," said Mr. Campion, of the group VeriSEAL.

Mr. Nalls' story shows that those who lie about being SEALs are often unstable and dangerous, said Mr. Campion, who graduated from Slippery Rock University in 1979 and then spent 20 years as a SEAL.

Military records show that Mr. Nalls served in the Navy, but he was a cook, not a SEAL. Based in Norfolk, Va., from 2001-04, he prepared meals for a helicopter maintenance crew.

After being honorably discharged, he returned to the Pittsburgh area and publicly claimed he had been a SEAL who took a bullet to the shoulder during combat in Fallujah, Iraq. He also lied about helping to rescue seven stranded soldiers.

Exposed as a fake in June 2005, he then was arrested in a string of crimes, including robbery, assault and drunken driving.

Police added domestic violence to his rap sheet in August 2006. They charged Mr. Nalls with shoving his girlfriend, Jobeth Olson, then eight months pregnant.

He killed Ms. Olson in April 2007 with a bullet to the back of the head.

SEAL "wannabes" lie about their military service because they crave attention or want to impress women, said Steve Waterman, a former Navy diver, parachutist and combat photographer in Vietnam, who also works with VeriSEAL.

The first Navy SEAL teams were formed in 1962 under President Kennedy's administration. Since then, the number of SEAL impostors who have been caught has far exceeded the number of men who actually served in the storied units, said Mr. Waterman, who has been hunting down phonies for more than a decade.

"We're approaching 10,000 fakes that we've found and documented at some level. They outnumber the actual SEALs," said Mr. Waterman.

Mr. Campion said impostors wrongly believe that Navy SEALs are steeped in secrecy, so they will not be caught. If questioned about specifics, the phonies typically say information about their SEAL graduating class is "classified," even though there is no prohibition against a SEAL revealing his military background, Mr. Campion said.

One such impostor, Ralph Ervin Crowder of Independence, Mo., served eight months in federal prison in 2003 for falsely claiming he received the Medal of Honor while serving as a SEAL.

Real SEALs include former U.S. Sen. Bob Kerrey of Nebraska, former Minnesota Gov. Jesse Ventura and the late Michael Monsoor, who in April received the Medal of Honor for uncommon valor in the Iraq war.

Notable SEAL impostors have included a Pennsylvania school superintendent, the director of military programs at Southern Illinois University and a part-time history professor at a college in Eastern Pennsylvania.

The professor was David Silbergeld, who was fired from Luzerne County Community College in 2002 for falsifying his job application, lying about a felony record and falsely claiming to be a SEAL. Mr. Silbergeld shot and killed himself in 2003.

During his murder trial, Mr. Nalls testified that his father was a SEAL. He said he mentioned this to a student, who misunderstood and thought he was talking about himself. The story spread, and Mr. Nalls said he never corrected the mistake.

But under questioning by a prosecutor, he admitted describing himself as a SEAL. "That was a lie," Mr. Nalls said.

His trail of deceit has more than one victim. He has a 20-month-old daughter with the woman he killed.