Wildlife officials are investigating the deaths of two Mexican gray wolves they say were killed after being struck by a field team's tranquilizer darts.
The team of state and federal wildlife officials was surveying the wolves in an annual population count that also involves capturing wolves with tranquilizer darts to attach radio collars to them.
In a statement Tuesday, the Arizona Game and Fish Department and the U.S. Fish and Wildlife Service said one wolf was darted Jan. 23 and released into the wild before dying four days later.
The other died Jan. 28 within minutes of being darted.
Officials say the wolves are the first to die during "count and capture" operations conducted since 2005.
About 110 Mexican gray wolves roam a portion of Arizona and New Mexico.
Necropsies are planned for the two wolves.