The bus was taking schoolchildren from Mark municipality in southwest Sweden on a ski trip to the mountains in the north. It had travelled almost 600km overnight when it crashed off the E45 road at 7am on Sunday.
One of the 23 injured, a 14-year-old girl, remains in a critical condition in hospital.
"It is routine in these sorts of events that one launches a preliminary investigation," prosecutor Karin Johnson Ryd said in a statement. "In this case I have come to the conclusion that the circumstances are such to merit suspicion of a crime."
The driver is also suspected of involuntarily causing bodily harm.
I want to spell out and underline that its early in the investigation, that this is the lowest grade of suspicion, and that a lot could change," Johnsson-Rydh told Expressen newspaper.
Also after the announcement, the Kommunal labour union announced that it was preparing a lawsuit against Bergkvara buss, the owner of the vehicle, in order to protect the driver.
"After the tragic accident, there has been a strong focus on the driver, who is one of our members, so we feel a responsibility in the debate being carried out in the mass media to put forward our view of the company," Johan Ingelskog, Kommunal segment chief, told TT newswire.
Bergkvara Buss is a company which we have for a long time had under scrutiny. Many of our members have reported overly packed schedules, stress and a generally poor working environment."
The bus driver is set to be questioned, but the prosecutor said no date or time has yet been set.
There were 59 people on board the bus, 52 young teenagers and seven adults.