An investigation is underway in east Wardak province in Afghanistan after a gun battle between US and Afghan forces left one US soldier, an American contractor, and three Afghan troops dead.
Officials had initially thought the incident was the latest in a series of so-called insider attacks but NATO now says insurgent fire may have been a factor.
53 NATO personnel have died in what are called "insider attacks" where Afghan troops turn on coalition forces they are working with.
This latest attack brings the number of American service personnel killed in Afghanistan since the war started to two thousand.
Al Jazeera's John Terrett has more.