General Motors chief executive Fritz Henderson resigned unexpectedly Tuesday from the government-owned company, once again unsettling the ranks at the nation's largest automaker as it struggles to right itself after bankruptcy. The change comes just months after Henderson's predecessor, G. Richard Wagoner Jr., was forced to resign by the Obama administration. The administration and the company said the government had no immediate role in Henderson's departure. Edward E. Whitacre Jr., the former chief executive at AT&T who was appointed by the government as GM board chairman earlier this year, will serve as interim chief executive as a search begins for a permanent replacement. 'While momentum has been building over the past several months, all involved agree that changes needed to be made,' Whitacre said in a brief statement read to reporters at the company's Detroit headquarters. 'We now need to accelerate our progress.' Whitacre described the decision for Henderson to resign as mutual but declined to answer questions. Henderson could not be reached for comment. His departure appeared to come suddenly. Henderson had been slated to make a major speech at the Los Angeles Auto Show on Wednesday, and earlier Tuesday, the company was issuing media advisories regarding his address. But the company board had been meeting in Detroit on Monday and Tuesday. It was there that the resignation was accepted, according to company officials. 'This was not supposed to have happened,' said Gerald Meyers, the former chairman of American Motors and now a professor at the University of Michigan. 'I'm not going to say it's unfair because there's nothing unfair about business. But this does seem to have been short-tempered.' Henderson had been chief executive for only about eight months. Meyers said it would have taken far longer than that, maybe 18 months, to fairly assess whether he 'could move the needle on sales and profits.' Exactly what precipitated Henderson's exit was not clear. Previously one of Wagoner's deputies, Henderson had been elevated to chief executive by the government's autos task force in March after it ousted Wagoner. Whitacre said that 'Fritz has done a remarkable job in leading the company through an unprecedented period of challenge and change.' But he declined to discuss what led to the breach. GM said the Obama administration did not play a role in the decision and was informed only after it was official. But the administration did appoint several of the board members. The government owns 61 percent of the automaker after it invested $50 billion in its rescue. 'This decision was made by the Board of Directors alone. The administration was not involved in the decision,' the Obama administration said in a statement. In recent months, Henderson and Whitacre have alternately stepped into the company spotlight, each melding the corporate identity with his own. Henderson started a blog called Ask Fritz, in which he fielded questions from customers. Whitacre, meanwhile, has introduced himself to national television audiences in the company's widely broadcast 'May the Best Car Win' television commercials. 'I'm Ed Whitacre, the new chairman at General Motors,' he tells the camera. 'Before I started this job, I admit, I had some doubts, probably a lot like you. But I like what I've found.' While Whitacre, 67, once claimed, 'I don't know anything about cars,' Henderson has worked almost his entire career at GM, and his father worked there, too. 'Ed is a larger-than-life individual,' said Jeremy Anwyl, chief executive of Edmunds.com. 'It could have been a matter of chemistry. But it's hard to look at the last 30 days and see what went wrong. In many ways, the company is making progress.' Henderson and the board, however, have reportedly differed on whether to sell the European division Opel and when to repay the government. And in recent months, the company has been frustrated in its attempts to sell the Saturn and Saab brands. The Saturn deal collapsed when the buyer backed out; the brand is being wound down. Similarly, a deal to sell Saab fizzled when a buyer withdrew; the fate of the Saab brand has not been decided. The U.S. auto industry has come under immense pressure because sales have collapsed, dropping from a rate of about 16 million a year to less than 10 million during the recession. U.S. sales of cars and light trucks in November came in at an annualized rate of 10.93 million units, according to Autodata, up slightly from 10.41 million units in November last year. Though GM has been shedding brands like Pontiac to streamline itself, the company has managed to hold on to more of its U.S. market share than expected. Its November market share was 20.1 percent, down from 20.4 percent in November of last year, according to Autodata. Moreover, some of GM's new cars have done well. 'We're outperforming our internal projections,' Michael DiGiovanni, GM's chief market analyst, said before the announcement of Henderson's resignation. 'We're encouraged.' The progress may not have been as rapid as the company's new board may have liked, however, and its impatience would have been expressed to Henderson. Whitacre is 'so tough, I can't think of anyone who would survive unless he was a deity,' said Meyers, who knows Whitacre and says he is an 'outstanding executive.' 'What Ed Whitacre wants in a CEO is Ed Whitacre,' Meyers said. It was the Obama administration's task force that chose Henderson. But Henderson's appointment was taken with some trepidation because he is a longtime GM executive, officials said. Ultimately, officials concluded that it would be less disruptive to elevate Henderson rather than conduct an executive search. But the questions about whether the humbled corporate giant could be reformed with an insider at the helm never abated. Former autos task force chief Steven Rattner insisted recently that the company still 'needs a real house-cleaning.' At a recent meeting with Washington Post reporters and editors, Henderson was asked whether it was a problem that neither he nor any of the people who report to him are new to GM. 'Yeah,' he said. But, 'almost all of them are new in their jobs, which I think provides an interesting perspective. We've changed a lot of people. ... I do think there is a benefit to bringing in some fresh perspective.' Staff writer Steven Mufson contibuted to this report. 