Updated at 10:55 a.m. with comments from Jeff Bjornstad:

WASHINGTON -- Jeff Bjornstad, Sen. Patty Murray's chief of staff who managed her successful campaign for a fourth term in November, is leaving her office.

Bjornstad's departure was announced to Murray's staff Thursday, Murray's spokeswoman, Alex Glass, confirmed. Bjornstad will be replaced by Mike Spahn, who now serves as Murray's aide in her capacity as the Senate Democratic Conference Secretary. Glass said Bjornstad's decision to leave was his.

Bjornstad, 43, is a University of Washington political science major who grew up in Everett. His father worked as a railroad switchman. While an undergraduate, Bjornstad interned for U.S. Rep. Adam Smith, who then was running for Washington state Legislature. Bjornstad rose to serve as Smith's chief of staff. Bjornstad later took the same job with U.S. Rep. Rick Larsen.

Then in early 2007, Murray called Bjornstad with a job offer when her chief of staff, Rick Desimone, left to become a lobbyist. Bjornstad is married to Murray's former state director, Christy Gullion, who is now the chief federal lobbyist for the University of Washington. The couple live in Washington, D.C.

Glass said Spahn will assume his new duties "in the next few weeks."

In an interview, Bjornstad said he'd had "wonderful four years" working for Murray. But he added that "now is a natural time to think about new challenges," noting that he has spent 14 years on Capitol Hill in addition to a half decade in Olympia.

Bjornstad said he is actively pursuing new opportunities but has no firm plans yet. He declined to say whether he would seek work as a lobbyist.

Bjornstad said the most fulfilling part of his job was "the all-day long problem solving."