Federal prosecutors have launched a criminal investigation into a legislative redistricting plan that a court threw out on the grounds that it marginalized black voters and protected white incumbents.

The focus of the investigation was unclear, but a citizens group had called for a perjury investigation after the federal court cast doubt on House Speaker Thomas Finneran's (search) sworn denial of his involvement in redrawing the map.

The map was used in the 2002 election but was rejected in February by the 1st U.S. Circuit Court of Appeals (search). Some of the 17 Boston House districts affected by the ruling are held by House leaders including Finneran and Majority Leader Salvatore DiMasi (search), both Democrats.

The grand jury subpoena for documents was sent March 10 to the office of Democratic state Rep. Thomas M. Petrolati, the chairman of the House redistricting committee that created the map, The Boston Globe reported Thursday.

"The committee will cooperate fully with that request," Thomas H. Hoopes, the panel's lawyer, said in a statement Thursday. "There will be no further comment of any kind."

Finneran's spokesman, Charles Rasmussen, referred all calls to Hoopes, who would not comment beyond his statement.

Finneran has testified that he was not involved in drawing the controversial map, but the court said his statement was not credible.

"Although Speaker Finneran denied any involvement in the redistricting process, the circumstantial evidence strongly suggests the opposite conclusion," the judges said in their ruling.

Common Cause Massachusetts, a nonpartisan citizens group, had asked federal prosecutors last month to investigate whether Finneran committed perjury in his testimony before the 1st Circuit.

Samantha Martin, a spokeswoman for the U.S. Attorney's Office, would not comment, saying federal law prohibits prosecutors from confirming or denying the existence of a grand jury investigation.

The appeals court, in rejecting the map, said it provided white voters with a number of majority-white districts, but failed "to provide African-American voters with a proportional number of majority black districts."

February's ruling barred the state from holding elections this fall in the House districts affected until a new map is drawn. The three-judge panel was scheduled to hold a hearing Friday to consider six alternative plans that have been drawn by plaintiffs and the House leadership.