WASHINGTON – Energy Secretary Steven Chu, who won a Nobel Prize in physics but came under questioning for his handling of a solar energy loan, is stepping down.

Chu offered his resignation to President Barack Obama in a letter Friday. He said he will stay on at least until the end of February and may stay until a successor is confirmed.

Chu's departure had been widely expected and follows announcements by Interior Secretary Ken Salazar, Environmental Protection Agency chief Lisa Jackson and Jane Lubchenco, head of the National Oceanic and Atmospheric Administration, that they are leaving.