A 12-year-old boy was cooking with his older brother before he shot his six-year-old brother dead, wounded the older brother and killed himself, a Florida sheriff says.

Pasco County Sheriff Chris Nocco identified the 12-year-old boy as Kevin Pimentel. Deputies say he killed six-year-old Brady Pimentel and wounded 16-year-old Trevor Pimentel.

The sheriff earlier said there was an argument over food, but later said that was under investigation.

He said the boys were home alone while their mother was working. Authorities have found no evidence that child welfare officials or police had been called to the home before the shooting last night.

Mr Nocco said the mother still does not know all of the details about the shooting.

"This is a horrific scene," he said. "This is a nightmare."