Two Florida brothers originally from Pakistan were indicted Friday, accused of plotting to use an explosive device and conspiracy to provide material support to terrorists.

Raees Alam Qazi, 20, and Sheheryar Alam Qazi, 30, were arrested by FBI agents in Fort Lauderdale on Thursday. The indictment does not provide specific details about what the men may have been targeting, saying only they conspired to use a "weapon of mass destruction" against people and property in the United States.

The indictment alleges that the Qazis engaged in their conspiracy from at least July 2011 until the time of their arrest. There is no mention of whether any explosives or other weapons were seized when the men were arrested.

A federal law enforcement official said Raees Qazi was the most active in the plot and of most concern to investigators. Raees had been in communication with people overseas and investigators are trying to determine whether any of those people have connections to terrorism, the official said.

The official stressed investigators do not know yet if the brothers were acting on their own or whether there was any direction from overseas, the official said.

"They were arrested now because we were very concerned he (Raees Qazi) posed a very serious threat," said the official, who was not authorized to discuss the ongoing investigation.

Unlike a number of recent terror arrests in the United States, this case did not involve an FBI sting operation in which undercover operatives supplied the suspects with inert explosives, the federal law enforcement official told CNN. The official described the younger Qazi brother as very intelligent and aware the FBI had infiltrated and broken up other plots, so the FBI did not attempt an undercover operation in this case.

A second federal law enforcement official said any possible attack the suspects had in mind was "not imminent." Neither official would provide details about what the men wanted to target.

The Justice Department also would not elaborate on the alleged plot. "The investigation is ongoing," said spokesman Dean Boyd. "Any potential threat posed by these two individuals has been disrupted."

The two men, who are naturalized U.S. citizens living in Broward County, had a brief court appearance Friday and were ordered detained at least until a detention hearing set for December 7.