member of Al Qaeda who was involved in plans to blow up the United States Embassy in Singapore is providing federal authorities with important new details about that plot and other aspects of the Qaeda terrorist group's operations, law enforcement officials said yesterday.The Qaeda operative, Mohamed Mansur Jabarah, is being questioned by members of the F.B.I.-N.Y.P.D. Joint Terrorist Task Force at a military base in the northeastern United States. The officials said Mr. Jabarah had been one of the most cooperative of the Qaeda members detained since the Sept. 11 attacks and was likely to testify at trials of...