Some police reports suggested that the bomber had been driving a minivan.
The suicide bomber drove his car into the gathering itself and blew himself up.
Several officers who survived the attack reported that the first bomber was driving a police car and wearing a uniform.
Describing the incident, he said the first suicide bomber had driven out of a gas station toward his car.
At least one bomber drove a car to the stadium and detonated it near the entrance.
Witnesses said there were two explosions then a brief pause before another bomber drove his vehicle into the building, a former police station.
On Thursday, a suicide bomber drove a car into a convoy carrying at least 20 marines and soldiers.
A policeman and four members of his family were killed when a suicide bomber drove into the policeman's home in Heit.
Three policemen were killed when a suicide car bomber drove at their patrol in the Karrada district.
The suicide bombers who struck a residential compound in November, for example, were driving a fake armed forces vehicle.