SAN DIEGO -- Flying low over the Sonoran Desert, Border Patrol agents spotted a skeleton in the brush.

The harsh terrain along the southern edge of Arizona is a busy trafficking corridor for illegal immigrants and the remains could have been from a person who died while trying to sneak into the United States. But busy Interstate 8 runs nearby and it also could have been a slain U.S. citizen, a suicide, a runaway.

"It's rarely a cut-and-dry decision," said Joe Brigman, spokesman for the Border Patrol station at Yuma, Ariz.

Human rights activists, however, say it's in the government's interest to keep its count low. They contend the agency tries to shave the number of deaths by excluding many skeletal remains, car-accident victims and bodies discovered by local law enforcement agencies.

"The American people have the right to know the human cost, the real human cost, of these policies," says Claudia Smith, a San Diego attorney with the California Rural Legal Assistance Foundation.

From documents and other clues found on the body spotted east of Yuma, agents concluded the victim was indeed an immigrant -- among 325 the Border Patrol counted in the fiscal year which ended Sept. 30, down from 340 the year before.

The counting method is inconsistent from one government agency to the next -- and sometimes even from one Border Patrol sector to another, said Claudia Smith, a San Diego lawyer with the California Rural Legal Assistance Foundation.

.For instance, in Arizona, the busiest stretch for illegal entries, the Border Patrol reported a record 177 deaths in fiscal 2004. But medical examiner's offices in Arizona put the toll at around 221.

Andy Adame, spokesman for the Tucson Border Patrol sector, said local law enforcement agencies do not always notify the Border Patrol when they handle the death of a possible illegal immigrant.

Agents in Tucson say they do not count the deaths of illegal immigrants' smugglers or guides.

"We count someone who is furthering their entry into the United States," Adame said in a telephone interview. "And we feel smugglers and guides are just going back and forth, so they aren't counted."

However, Border Patrol officials in Washington, D.C., say it has been standard policy to include smugglers since 1998.

Location also affects the count.

Smith points to the case of 19 people found dead in a stifling truck trailer near Victoria, Texas, in 2003. It was the deadliest immigrant-smuggling attempt in the United States in more than 15 years, but the deaths were not included in Border Patrol statistics, officials say, because the victims were too far from the border.

The agency's count typically includes people found only in 43 U.S. counties that are within a 100-mile-wide belt along the border with Mexico.

Border Patrol officials bristle at the allegation they are undercounting deaths.

"We are very interested in knowing about any death discovered along the border," said Mario Villareal, a Washington-based spokesman for Customs and Border Protection.

"Unfortunately, there are deaths that occur every day throughout the country," Villareal said. "Making a determination about a possible migrant death 200 or 300 miles from the border is outside our operational authority."

The agency also only counts deaths of migrants found on the U.S. side of the border. Mexico tracks fatalities on either side of the border, but only those of Mexican nationals.

In 2004, Mexico counted 286 such deaths. In each of the previous three years, Mexico's death toll was higher than the total kept by the Border Patrol.

Some activists are calling for an independent agency to take charge of counting the dead.

"We're entitled to know the human cost of these (border control) programs," said the Rev. Robin Hoover, founder of Humane Borders, an organization that has set up water stations in the Sonoran desert for border crossers. "How can we possibly devise proper public policies if we don't even have an accurate body count?"

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On the Net:

Customs and Border Protection: http://www.cbp.gov/

California Rural Legal Assistance Foundation: http://www.crla.org/

Humane Borders: http://www.humaneborders.org/