The mysterious midnight blast which rocked the Yard on Wednesday was last night termed an "inside job" by Alvin R. Randall, chief of the University police.

Although the exact nature of the "bomb" which caused the Thayer Hall disturbance has not been determined, Randall believes that dynamite was the chief ingredient in the missile which shattered a first floor window.

After questioning students and proctors who live in the Hall, Randall said that all the information points to a "student prank," something which might have been serious but wasn't. "As it is," he added, "the person or persons who caused the explosion were probably more scared than anyone else."

Though admitting that the police have no leads to work on, Randall declared that "We never give up." Reports in a Boston newspaper that the incident was one of a series involving both students and Cambridge residents were denied by Randall. "As far as I know," he said, "there has been no trouble of that nature this year."

Promise Punishment

If the pranksters are apprehended they will be turned over to the Dean's Office for disciplinary action, Randall revealed. "I'd hate to think what would happen to them if they were discovered," he noted.

The only other incidents of the kind that Randall could recollect were the explosion near the Charles River in the fall of 1943 and the attempt to mine Soldiers Field by a group of MIT students before the Yale game that year.

The explosion, which occurred but a few moments before midnight, completely destroyed the window of the room in which Robert W. Dobble '53 was sleeping. He was covered with glass fragments but escaped unhurt except for a cut on one arm.