WASHINGTON (Reuters) -The top federal prosecutor who supervised criminal cases at the Washington, D.C. U.S. Attorney's office resigned on Tuesday, citing what she described as an improper demand by officials appointed by President Donald Trump's administration to launch a criminal probe and pursue an asset freeze. In a letter to the interim U.S. Attorney Ed Martin reviewed by Reuters, the office's criminal chief Denise Cheung wrote that the Trump administration had ordered her to investigate a government contract awarded during Joe Biden's administration and pursue a freeze of the recipient's assets. "I have been proud to serve at the U.S. Department of Justice and this office for over 24 years," she wrote in her letter.