Clark County Sheriff Douglas Gillespie and other officials didn't immediately identify the police officer or the family members, pending positive identification and the release of names by the Clark County coroner.
"There was an incident today involving one if Metro's lieutenants," a somber Gillespie said in a 2 1/2-minute statement to reporters at a hastily called news conference. "Several bodies were discovered."
Coroner Michael Murphy said after investigators left the fire-charred home Monday afternoon that identifications would probably be made Tuesday.
Gillespie, the elected head of the Las Vegas Metropolitan Police Department, said the lieutenant was a 20-year Las Vegas police veteran. Gillespie spoke of unanswered questions and "untold grief" for family members, friends and co-workers. He took no questions.
The sheriff said the investigation in Boulder City, about 20 miles southeast of Las Vegas, was being handled by police in neighboring Henderson.
Henderson police spokesman Keith Paul said a man called 911 at about 8:20 a.m. Monday and told a dispatcher he killed his wife and child, set his house afire and would shoot anyone who approached.
The home is owned by Hans Walters, according to Clark County assessor records. Many know Walters as a Las Vegas police lieutenant married to a former Las Vegas police officer, Kathryn Walters. The Las Vegas Review-Journal reported that she left the department in 2005.
Boulder City police arrived to find the homeowner with a handgun in the doorway of the burning house before he retreated back inside and apparently killed himself, Paul said.
No shots were fired by police or SWAT officers from Henderson and North Las Vegas who later found the bodies of a 52-year-old man, a 46-year-old woman and a boy inside, Paul said. Officials said the boy was believed to be about 7.
"We're investigating the incident as a murder-suicide at this time," Paul said.