The number of deaths among illegal immigrants crossing the Arizona desert from Mexico is soaring so high this month that the medical examiner's office that handles the bodies is using a refrigerated truck to store some of them, the chief examiner said Friday.

The bodies of 40 illegal immigrants have been brought to the office of Pima County Medical Examiner Dr. Bruce Parks since July 1. At that rate, Parks said the deaths could top the single-month record of 68 in July 2005 since his office began tracking them in 2000.

"Right now, at the halfway point of the month, to have so many is just a very bad sign," he said. "It's definitely on course to perhaps be the deadliest month of all time."

From Jan. 1 to July 15, the office has handled the bodies of 134 illegal immigrants, up from 93 at the same time last year and 102 in 2008. In 2007, when the office recorded the highest annual deaths of illegal immigrants, 140 bodies had been taken there through July 15.

Parks said his office, which handles immigrant bodies from three counties, is currently storing roughly 250 bodies and had to start using a refrigerated truck because of the increase in immigrant deaths this month.

He said many of the bodies seem to be coming from the desert southwest of Tucson, where it tends to be hotter than eastern parts of the border or the Tucson metro area.

Authorities believe the high number of deaths are likely due to above-average and unrelenting heat in southern Arizona this month and ongoing tighter border security that pushes immigrants to more remote, rugged and dangerous terrain.

Erik Pytlak, a meteorologist with the National Weather Service, said Tucson's average nighttime lows in the first 15 days of July are the hottest for that period in recorded history.

If nighttime temperatures don't cool down enough, the human body doesn't get a break from the daytime heat, which has hit 109 degrees in recent days in southern Arizona.

"Instead of having one day of a lot of heat, you have day after day after day, and you have a steady stream of people in the desert � people start succumbing, unfortunately," Pytlak said.

He said if possible thunderstorms materialize over the weekend, that could lower temperatures. But if rain doesn't fall and there's cloud cover, the situation could get worse because clouds hold temperatures up at night, he said.

While the bodies that go the Pima County Medical Examiner's Office don't represent all the deaths on the Arizona border, the Border Patrol also is seeing the effects of the weather.

"It does seem like the heat is really having a pretty significant impact right now," Agent Colleen Agle said.

Agle did not have statistics for July but said agents have been seeing "quite a few" deaths that appear heat-related.

On Thursday, she said the Border Patrol responded to a call from a husband and wife from Mexico who were experiencing difficulties because of the heat. When agents got there, the 25-year-old man had died; his 22-year-old wife survived and will be taken back to Mexico.

"Unfortunately, (agents) just didn't get there in time," she said, adding that finding immigrants in distress is often extremely difficult because of the vast and treacherous terrain. "It's really sad when this happens. Even one death is too many for us."

Deaths among immigrants occur despite public service advertisements warning them of the dangers of the desert, and the efforts of humanitarian groups that man aid stations for immigrants in distress and 20 Border Patrol rescue beacons in remote areas of the desert that immigrants can activate if they need help.

Border Patrol statistics for the entire U.S.-Mexico border show that deaths among illegal immigrants peaked at 492 in fiscal year 2005 and declined every year until last fiscal year, when they rose to 422.

The Border Patrol says the agency rescued nearly 1,300 people last fiscal year.