Nearly two dozen federal agents yesterday raided the Washington headquarters of the agency that protects government whistle-blowers, as part of an intensifying criminal investigation of its leader, who is fighting allegations of improper political bias and obstruction of justice

Bloch, who was nominated to his post by President Bush in 2003, is the principal official responsible for protecting federal employees from reprisals for complaints about waste and fraud. He also polices violations of Hatch Act prohibitions on political activities in federal offices.

Retired FBI agents and former prosecutors called the raid an unusual, if not unprecedented, intrusion on the work of a federal agency.

Bloch repeatedly clashed with his own workforce and with other Bush administration officials he targeted for improper behavior. By his own account, the White House twice asked him to resign.