Ten French investigators, including a judge and scientific and forensic police, arrived in Cambodia earlier this month to probe the deaths of widower Laurent Vallier, 42, and his young children.
"This (investigation) has led to very significant breakthroughs which are now ruling out the possibility of a suicide," the French embassy in Phnom Penh said in a statement.
The family's badly decomposed bodies were discovered inside Vallier's white 4×4 vehicle after it was retrieved from a large pond behind his house in southern Kampong Speu province in January last year.
Vallier and his two sons and two daughters, thought to have been aged from two to nine, had been missing since September 2011.
Chhim Rithy, a Cambodian investigating judge at Kampong Speu who was working with the team, said they had found some blood stain stains inside Vallier's house and on a rope.
"It is not the case of suicide. It could be a murder case," he told AFP, adding investigation in the case was still ongoing.
Vallier, who according to his relatives worked as a tour guide, is understood to have moved from France to Cambodia around 13 years ago, arriving in Kampong Speu in 2007. His Cambodian wife died in childbirth in 2009.