The FBI quickly arrested four radical Muslims, who were convicted in 1994. More members of the radical group were tried beginning January 16, 1995, for a wide-ranging plot of terrorist attacks.
One reason the FBI was able to act so quickly is that an FBI informant was the one who built the bomb. The U.S. government paid the informant, Emad Salem, $1 million for his testimony. Salem tape-recorded conversations with the bombing suspects. Unbeknown to the FBI, Salem also recorded his conversations with them.
The FBI benefited greatly from the World Trade Center bombing. In particular, the bombing resulted in the proposal of the 1995 Counterterrorism Bill greatly expanding federal authorities' budgets and powers.
From a December 1993 post by Terry Atwood:
"Tucked away on page A5 of the Washington Times for Wednesday, December 15 is a tiny article about tape transcripts of a conversation between the FBI informant, Emad Salem, and his controller, FBI agent John Anticev. As you may already know, Salem is a former Egyptian army officer hired by the FBI to infiltrate (or organize?) a terrorist group here in the U.S. In April, two months after the bombing, when the taped conversation took place, Anticev asked Selem to justify his expenses. Salem defended his expense report by saying that his usual expenditures were pushed up by the costs of building the trade center bomb. He acknowleged procuring the materials (at government expense) and personally building the bomb."
Sources: March 8, 1995, San Francisco Chronicle article, "New York Terror Trial Witness Tells What a Liar He Was," and post by Terry Atwood.