Bomb disposal experts arrived at the scene on Tuesday morning, as residents living near the garage were evacuated.
Onal said: "The meticulous work of the police averted a possible catastrophe... I do not even want to think about what would have happened if the attack had succeeded."
Bombers linked to al-Qaeda hit Istanbul in 2003 with truck bombs, killing 58 people in attacks that had targeted two synagogues, the British Consulate and a British bank.
In February, a court sentenced seven people to life in prison for the bombings.