Thirty-three members of Congress have written Attorney General Alberto Gonzales demanding that the FBI update lawmakers on the investigation into the anthrax attacks five years ago that paralyzed the nation with bio-terror fears.

The bipartisan letter escalates efforts by Sen. Charles Grassley, R-Iowa, and Rep. Rush Holt, D-N.J., to get the FBI to tell lawmakers what it has learned during the five-year case that remains unsolved. The FBI has refused, citing concerns about possible leaks.

The lawmakers said any leakers of prior information about the anthrax case inside the FBI or Congress should be punished but that such concerns do not justify keeping information from lawmakers so they could perform their required oversight of the FBI's performance.

The case remains unsolved five years later.

"As an institution, Congress cannot be cut-off from detailed information about the conduct of one of the largest investigations in FBI history," the lawmakers wrote. "That information is vital in order to fulfill its Constitutional responsibility to conduct oversight."

The letter is signed by members of both parties and members of both chambers of Congress including the outgoing chairman of the Senate Judiciary Committee, Sen. Arlen Specter, R-Pa.

Sen. Patrick Leahy, the panel's top Democrat and its next chairman, was one of the targets of the attacks in fall of 2001, getting an anthrax-filled letter sent to his office.