Energy Secretary Federico Pena, the highest-ranking Hispanic in the Clinton administration, is quitting in June, aides and White House officials said Monday.

Pena, who served as Transportation Secretary in Clinton's first term, submitted his letter of resignation to President Clinton Monday morning. Pena was scheduled to discuss his decision at a late morning news conference.Pena, 51, gave no reason for his departure, although he had expressed an interest in leaving Washington when Clinton finished his first term in early 1997. The former Denver mayor agreed to take over the sprawling Energy Department for one year.

An aide said he made the decision to resign over the weekend. The aide, who asked not to be identified, refused to discuss Pena's motive for leaving.

"He needs to explain this in his own words," the aide said.

At the Transportation Department, Pena ordered small airlines to follow the same strict safety rules as major carriers. He also sharply reduced the size of the giant agency.

But Pena was widely criticized for defending ValuJet after a 1996 crash in the Florida Everglades. The airline was later grounded for a time because of concerns about its maintenance operation.