A suspicious package left by a prison guard's mailbox exploded and seriously burned him when he opened it, authorities said Sunday as they worked to determine who sent it and why.

The 52-year-old officer was severely injured, Gov. Andrew Cuomo said, in an explosion that stunned the small central New York town of Floyd. Cuomo called the blast "horrific," and local, state and federal agencies were investigating and trying to determine whether the blast was related to the victim's work.

"He's in critical condition, but we keep fingers crossed and you pray for the best," the Democratic governor said.

Cuomo said the officer is getting "the best care he can get" at the burn unit of Upstate University Hospital in Syracuse.

"There are a lot of unanswered questions," Oneida County Sheriff Robert Maciol told reporters at a news conference. "And we're confident that with all of us working together, we can figure out who did this."

The corrections officer picked it up the package around 8 a.m. near his mailbox, Maciol said, saying he couldn't give specifics about the explosive. Cuomo said the item blew up as the victim opened it.

Authorities aren't releasing the man's name. He's hospitalized in stable condition with burns on his hands and arms, according to Maciol.

The state Department of Corrections and Community Supervision referred inquiries to the governor's office.

The explosion shook neighbors in the quiet town of about 3,800, a bedroom community for the old industrial cities of Rome and Utica. It's also near a prison, in Marcy.

"I heard the loud boom, and I was scared," resident Patricia Westcott said by phone. "It did sound like a bomb."

She opened her door and heard screaming from a few doors away, then sirens.

Authorities used automated phone calls to warn some residents to stay indoors as they searched the neighborhood with dogs and other means to make sure it was safe. Maciol said they concluded no other residents were in danger, but part of the road remained closed to most traffic hours later so investigators could keep gathering evidence.