William D. Pease, the government's chief prosecutor of Mary Treadwell, spent nearly four years investigating the allegations surrounding P.I. Properties' ownership and management of the Clifton Terrace apartment complex in Northwest Washington.

Pease, a short, bespectacled man, is a veteran prosecutor, having spent 13 1/2 years in the U.S. Attorney's Office here. Now that the trial is over, Pease himself will be leaving: while investigating the Treadwell case, he was promoted to an administrative job as deputy director of the U.S. attorney's operations in D.C. Superior Court.

Pease is also one of three local prosecutors under consideration by the White House to fill a vacancy on the D.C. Superior Court bench.

His new job will be something of a homecoming for Pease. When he first went to the U.S. Attorney's Office, he prosecuted everyday criminal cases in Superior Court and, during that period, won convictions in 1974 against the three killers of Gail A. Cobb, the first D.C. policewoman shot to death in the line of duty, in 1974.

Since 1977, Pease, now 40, has worked in the fraud section of the prosecutor's office and started his investigation of Treadwell and other P.I. Properties' officials after a 1979 Washington Post series disclosed the wrongdoing involving the Clifton Terrace project.

Pease, a graduate of the University of Maryland and the Boston University law school, is a tough, facile courtroom tactician, a lawyer given to asking barbed questions as a hostile witness squirms in the witness chair. Outside the courtroom, Pease employs a quick wit at times, while, at others, he exhibits a sharp and profane temper.