TUCSON, Ariz. (AP) - Temperatures have soared during the past week's run-up to summer in the Southwest, and deaths among migrants crossing Arizona's desert regions are also up - a potential ill omen, a border county medical examiner says.

Authorities recovered the bodies of 12 suspected illegal immigrants during the past week, a period that saw temperatures top 100 degrees each day in southern Arizona, said Pima County Medical Examiner Dr. Bruce Parks. His office performs autopsies on many of the scores of illegal immigrants who die crossing into Arizona each year.

One was killed in a car crash, but four died of heat exposure, Parks said. The causes of death for the other seven is not yet certain.

"I expect that the majority of those, if not all of those pending, are going to turn out to be heat-related," Parks said. "That's quite a few in a week's period of time, and it doesn't bode well for the rest of the summer if that trend continues."

Arizona is the busiest location for illegal entries on the Mexico-U.S. border, and except for the last fiscal year, the harsh Arizona desert has taken an increasing toll in lives of undocumented immigrants. Pima County is at the center of migrant routes, and the medical examiner here also covers deaths in two nearby counties, Santa Cruz and Pinal.

Typically, the deaths spike during the hottest months, from May through August, when triple-digit temperatures are normal. Agents in the Border Patrol's Tucson sector recovered the remains of a record 72 illegal immigrants in the desert in July 2005. Nearly all died from heat exposure, Parks said.

The nine most recent discoveries of bodies occurred Tuesday, Wednesday and Thursday, according to agency spokesmen.

They included:

- The body of a Mexican man found on the Tohono O'odham Indian Reservation. The man died after he fell behind at least five other men who had been walking for 24 hours without water.

- Human remains believed to have been lying in open desert for several weeks were found about eight miles east of the Naco port of entry. Authorities weren't able to determine the unidentified person's gender.

- A Guatemalan woman found near the village of Artesia, also on the Tohono O'odham reservation.

- An unidentified body discovered south of Sells on the reservation believed to have been in the desert for about two days.

- A man suffering from heat exhaustion who was left behind by his uncle and three others walking in the desert west of Arizona 286. He died six hours after being airlifted to a hospital.

- A man found by a law enforcement helicopter on the Tohono O'odham reservation.

- A 38-year-old woman from Ixtapa found dead northwest of Arivaca Junction who had been left behind by her brother because she was sick.

- An unidentified Mexican man found under a tree in a decomposing state west of I-19 about 17 miles north of the Mexican border.

"It does appear at this point that we're not seeing any change in the amount of people attempting to cross," spokeswoman Dove Haber said. "We're not seeing a slowdown, not at this point."

Parks said it is hard to predict but that the recent deaths could portend a similar jump in numbers over the rest of the summer.

Since the Border Patrol's fiscal year began Oct. 1, the Pima County medical examiner's office has counted 118 desert deaths in Pima, Santa Cruz and Pinal counties, Parks said. It remains unclear exactly how many of those were heat related.

"One of the biggest causes of death that we end up with each year is `undetermined,' where the pathologist doesn't feel that he has enough information to determine cause," Parks said. "I think that most of those are heat related, but decomposition is so advanced that you can't tell."

In other Arizona counties abutting Mexico, nine confirmed heat deaths have been counted since Oct. 1 among migrants.