But authorities are offering no details about who might have killed them or why.
At least three of the victims suffered stab wounds or blunt-force injuries and all four were homicide victims, Police Chief Cathy Lanier said yesterday.
The police chief said the injuries occurred before the fire was set on Thursday afternoon.
Lanier identified the couple as Savvas Savopoulus, 46, and his wife, Amy Savopoulos, 47. She said the other two victims had been tentatively identified as the couple's 10-year-old son, Philip, and a housekeeper, Veralicia Figueroa, 57, of Silver Spring, Maryland.
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Police were seeking information about a blue 2008 Porsche belonging to the family that was found on Thursday night in Prince George's County, Maryland. The car had also been set on fire, Lanier said.
There was no sign of forced entry at the house, Lanier said. The federal Bureau of Alcohol, Tobacco, Firearms and Explosives was assisting in the investigation.
Savvas Savopoulos was the president of American Iron Works, a building materials manufacturer based in Hyattsville, Maryland. A person who answered the phone at the company's headquarters declined to comment yesterday, and a message left after Savopoulos was identified was not immediately returned.
The family home, valued at USD 4.5 million, is located blocks away from Vice President Joe Biden's official residence and from the Washington National Cathedral, which was preparing to host memorial services for the victims.
The couple's son attended St Albans School, a private school adjacent to the cathedral, where counselors were on hand to assist grief-stricken students and staff.
"Our school community is grieving over this tragic situation at the home of one of our young students," Vance Wilson, headmaster of the school, said in a statement. Lisa Lambert, who lives nearby but did not know the family well, said the slayings have residents frightened.