Police arrested Daron Wint, 34, late on Thursday and believe he held and tortured members of the wealthy Savopoulos family and their housekeeper before killing them.
Five others were arrested at the same time, but only Mr Wint faces charges.
The arrest, in Washington, followed a week-long manhunt.
Savvas Savopoulos, 46, chief executive of American Iron Works, was found dead in his family's $4.5m (£2.9m) home in Woodley Park, northwest Washington, just blocks away from the home of Vice-President Joe Biden one week ago.