Bertil Stroberg, a former Swedish air force officer who was convicted of spying for Poland during the Cold War but always maintained his innocence, has died. He was 79.

Stroberg's wife, Marianne Stroberg, said Wednesday he died in Stockholm on March 25 following a yearlong battle against cancer.

Stroberg was sentenced to six years in prison for spying in 1983, but released on parole after serving three years.

The key evidence in his case was a letter the prosecution said he had written to the Polish embassy offering to sell military secrets. Handwriting experts failed to link the letter to Stroberg.

The letter was signed Sven Roland Larsson and asked that money be sent in that name to the Central Post Office. Stroberg was arrested in 1983 when he went to the post office to collect Larsson's mail.

The court rejected the officer's claim that he was the unwitting victim of a conspiracy and that he had picked up the letter as a favor for an acquaintance.

After a 2009 documentary presented new information about the case, Stroberg requested a retrial, but Sweden's Supreme Court dismissed the request in October 2011.

Marianne Stroberg said her husband was very disappointed by that decision.

"It was as if he lost all his powers then," she told The Associated Press. "He took it very, very hard."

"He was a man who worked hard and always did the right thing in life," she added. "He never even got a parking ticket."

Stroberg is survived by Marianne, two children and four grandchildren. His wife said the funeral would be private.