The U.S. Justice Department has lost almost 30 percent of itstax prosecutors in the past month, slowing a U.S. crackdown onoffshore banks that enabled tax evasion, according to four peoplefamiliar with the matter.

Twenty-five of the 95 prosecutors in the tax division leftheadquarters in Washington for six-month “details” with U.S.attorneys around the country, and another three took permanentassignments, according to the four people, who declined to beidentified because they aren't authorized to speak publicly.

Many of the lawyers handled cases involving foreign banks orfinancial advisers suspected of helping U.S. clients cheat ontaxes, the people said. The transfers came amid criminal probes ofat least 11 Swiss financial institutions, including Credit SuisseGroup AG, with the tax division leading or assisting eachprosecution.