It was unclear whether General Motors' board or CEO Fritz Henderson, above, frustrated by second-guessing, initiated his departure from the company.

DETROIT — The leader of the new General Motors was done in by an old problem at the nation's largest car maker: Change wasn't happening fast enough.

GM's board and CEO Fritz Henderson parted ways Tuesday, the board upset that the automaker's turnaround wasn't moving more swiftly and Henderson frustrated with second-guessing, two people close to the former CEO said.

Board Chairman Ed Whitacre Jr., the former head of AT&T, will take over as CEO while a global search is conducted.

It was unclear whether Henderson or the board moved first in the surprise resignation, which came just hours before Henderson was to be the high-profile keynote speaker at the Los Angeles Auto Show. At a hastily called news conference at General Motors's downtown Detroit headquarters, Whitacre would not answer questions, but said the board and Henderson agreed that he should step down.

Whitacre thanked Henderson, 51, a lifelong GM employee, for his leadership and said the company is on the right path toward offering high-quality cars and trucks worldwide.

Both men were chosen for their jobs by the U.S. government, which owns more than 60 percent of the Detroit automaker in exchange for giving it billions in loans. But Henderson is a GM insider, while Whitacre is an outsider to the whole industry. An Obama administration official said in a statement "this decision was made by the Board of Directors alone. The Administration was not involved."

"I don't think this has much to do with Fritz Henderson's performance, I think it's just the wrong time to be a GM lifer," said Logan Robinson, a former Chrysler attorney and professor of corporate governance at University of Detroit Mercy.

Henderson, who rose through GM's ranks over a 25-year career, took over in March after the government forced out former CEO Rick Wagoner. A few months later, GM entered bankruptcy protection and Henderson led the company through a painful government-led and court-supervised reorganization. The company emerged from court protection in just 40 days cleansed of massive debt and burdensome contracts that would have sunk it without roughly $52 billion in federal loans.