Accidentals deaths during capture-and-count operation add to significant losses from poaching to the endangered species.
Federal and state wolf biologists in Arizona and New Mexico have temporarily halted an annual capture-and-count operation after the deaths of two endangered Mexican gray wolves that they shot with darts.
The U.S. Fish and Wildlife Service announced Tuesday that two female wolves died in late January.
One was killed immediately after it was darted from a helicopter with a tranquilizer; the other died a few days after it was captured, fitted with a transmitting collar and released.
The agency said the operation had used the same drugs and handling guidelines used successfully during the past two years and that 13 other wolves were successfully captured and released this winter.
The dead wolves will undergo necropsies at a federal laboratory in Oregon to determine the causes of death.
"This team of biologists is dedicated to the recovery of these wolves," Mexican Wolf Recovery Coordinator Sherry Barrett said in a written statement, "and the last thing they want is to have a wolf die during these operations."
The two accidental deaths add to a year of losses that could include a record number of illegal shootings — 15 of them, according to a state wolf biologist; the whereabouts of other wolves are unknown.
M1342, a male wolf whose capture and release near Alpine was featured in an Arizona Republic article last week, was not among the dead.
Federal and state employees capture some wolves in eastern Arizona and western New Mexico each winter while making aerial surveys of a wild population that, at last year's count, numbered 110. They attach transmitting collars to track the wolves and help manage conflicts with livestock, and they treat the anesthetized wolves with vaccines and anti-parasite medications.
The Fish and Wildlife Service is expected to release an estimate of this year's wolf population next month.
The population experienced strong pup production last spring, officials say, potentially offsetting the losses.
Mexican wolves, a subspecies of the northern gray wolf and a native of Arizona and New Mexico, were hunted nearly to extinction in the wild before a captive-breeding program took in the last few and enabled a 1998 reintroduction into the wild. They remain under federal protection.