DETROIT — A Swedish specialty automaker backed out of a deal to buy Saab from General Motors, casting serious doubt on the future of the troubled brand.

GM said today Koenigsegg Group AB decided to end the deal, which was announced in June. Financial details of the Saab acquisition were not disclosed by GM.

GM had been trying to unload the Swedish Saab brand as it restructured under Chapter 11 bankruptcy protection earlier this year.

GM Chief Executive Fritz Henderson said the Detroit automaker is disappointed in the decision and will take the next several days to figure out what to do.

A person briefed on the deal said today Koenigsegg informed GM of the decision Monday, and Saab's future is now unclear. GM's board will have to decide the company's next move, said the person, who asked not to be identified because the decision has not been made.