Investigators from the New York City Police Department, the Federal Bureau of Investigation, and the Bureau of Alcohol, Tobacco, and Firearms conducted an exhaustive initial search that produced the first clues. They determined that, because the blast took place in the garage, the explosive device was probably a car bomb. The recovered vehicle identification number was quickly traced to a Ford Econoline van which had been rented to Mohammad Salameh. Once in custody, Salameh's clothing and the rental agreement he signed for the van were tested for traces of explosives. Investigative work subsequently led to other suspects, although the evidence against Salameh helped most in the investigation. The more investigators looked into Salameh's activities, the more they became convinced he was part of a conspiracy. The case against another defendant, Nidal Ayyad, was almost as solid as that against Salameh but was developed using more technological methods. The cases against two other defendants, Mahmud Abouhalima and Ahmad Ajaj, were relatively weak. Evidence surrounding another suspect, Ahmed Yosef, helped solidify the cases against the others.