The terrorist's van carried not only the bomb, but the son of Michael Faraday. When he saw the bomb and the boy together, together with the remembered conversation from Oliver Lang about how Faraday's dead wife's friends would pay, he was lead to believe the bombing target was the FBI building. He followed the van because he saw the bomb placed aboard, knew the target, and that the bombing would be imminent.
His chase was interrupted by Oliver's crashing into him (because the van lead Faraday right into the alley where the "accident" could occur), and a fight ensued in the warehouse, where (off-screen) the bomb was transferred from the van to Michael Faraday's car. When Faraday escaped the warehouse and continued on to the FBI building, he encountered a different van from the same company on its daily delivery run to the FBI building. He would not have known about the daily delivery, but since the company was used by Lang and his co-conspirators, Oliver certainly did (remember during the fight he screams "Do you really think we'd leave anything to chance!?!"). Counting on the mistaken identity, and Michael's crazed chase to save his son, everything carries into the FBI building. Faraday still believed, passionately, his son and the bomb were now in the FBI garage, in the van.
He was wrong on both counts, the son was with the terrorist's wife and the bomb was in his own car. We see this milliseconds before the blast. Delivery and execution of the bombing complete, the terrorists go away scot-free with a patsy to boot.