The accident, which occurred early Friday in eastern France, killed two sisters aged 35 and 41, and three children aged from seven to 13. The family had been driving across France to a summer vacation spot.
The father of one of the children and boyfriend to one of the women committed suicide early Saturday, a source close to the investigation said.
The prosecutor, Remi Coutin, told AFP that tests on the 35-year-old driver, who was hospitalised with serious injuries, showed "such a quantity of cocaine that we can deduce to a probable degree that drug use occurred several hours before he sat behind the wheel".
Tests for other substances including alcohol turned up negative.
The prosecutor said an expert was going to be called in to determine what effect taking cocaine could have had on the truck driver.