Keep in mind while reading: This is from an ULTRA-liberal staff writer (completely unbiased) at the San Fran Chronicle.

Leaders of San Francisco's police officers union have accused Weather Underground co-founder Bill Ayers and his wife, Bernardine Dohrn, of taking part in the 1970 bombing of a city police station that killed a sergeant.

The union leveled the charge in a letter to a conservative organization lobbying for arrests in the case, but said it had not been in contact with investigators and had no new evidence related to the bombing, which killed Sgt. Brian McDonnell.

Instead, the union cited information from a former Bay Area resident, Larry Grathwohl, who is working with the conservative group, America's Survival Inc. of Maryland. Grathwohl asserts that he infiltrated the Weather Underground as an FBI informant and heard Ayers confess to a role in the bombing.

Ayers has denied any involvement in the bombing and, in January, called Grathwohl a "paid dishonest person" in an interview with The Chronicle.

The union's accusation surprised some authorities. According to a source familiar with the probe, who spoke on condition of anonymity, investigators have found no evidence that links the Weather Underground to the bombing.

America's Survival is planning to hold a press conference today at the National Press Club in Washington to discuss the case and the union's letter.

"There are irrefutable and compelling reasons to believe that Bill Ayers and his wife Bernardine Dohrn ... are largely responsible for the bombing of Park Police Station," the letter says.

The letter, dated Feb. 24, is signed by union President Gary Delagnes, Vice President Kevin Martin, Secretary Tony Montoya, Treasurer Martin Halloran and Sergeant at Arms Christopher Breen.

Ayers, an education professor at the University of Illinois at Chicago, did not return a call Wednesday. Dohrn, a law professor at Northwestern University, was out of the country.

Martin said he wrote the letter after he was contacted a few weeks ago by Cliff Kincaid, the nonprofit group's president. Martin said he trusted Grathwohl, a Cincinnati resident who once lived in Castro Valley.

"It's coming directly from a person who had close, confidential conversations with Ayers and members of the Weather Underground," Martin said. "We have no reason to doubt his assertions."

Martin said the union wanted to "bring this case to attention, especially because Bill Ayers has been on a speaking tour. His actions have elicited this type of response."

The attack happened Feb. 16, 1970, when a bomb placed on a window ledge of Park Station at Waller Street and Kezar Drive killed McDonnell and injured eight other officers. The case got attention two years ago when prosecutors charged a group of alleged former Black Liberation Army members in the 1971 shooting at the Ingleside Station that killed Sgt. John Young.

A grand jury that investigated the Ingleside case in 2005 also looked at the Park Station bombing, but the results of the probe were not released. No one has ever been charged with McDonnell's death.

Grathwohl said Wednesday that he infiltrated the Weather Underground in late 1969, first at the urging of Cincinnati police and then at the direction of the FBI.

He said that in early 1970, Ayers visited him and other operatives in Buffalo, N.Y., and said Dohrn had been forced to plant the bomb at Park Station because others weren't active enough in committing violence. Ayers also knew the composition of the bomb and where it had been set, Grathwohl said.

The Weather Underground took responsibility for bombing the Pentagon and other buildings in the 1970s. Ayers and Dohrn were indicted in 1970 after a bomb blast killed three members in a New York City townhouse. Charges were later dropped because of misconduct by investigators.

Ayers became a lightning rod for conservatives during last year's presidential campaign, as some faulted Barack Obama for the two men's ties in Chicago education circles.

In addition to running America's Survival, Kincaid is the editor of a conservative organization called Accuracy in Media. He has attempted to link Obama to communism and socialism.

He also has views that might be considered extreme in San Francisco, such as calling for a "quit gay sex" campaign. "It appears that the homosexual lifestyle is as addictive as smoking," he once wrote.

Kincaid said he was not going after Ayers to hurt Obama, but added, "We are concerned that we now have an attorney general who was involved in Clinton administration pardons of members of the Weather Underground."

E-mail Demian Bulwa at dbulwa@sfchronicle.com.