The six-month election recount that turned former "Saturday Night Live" comedian Al Franken into a U.S. senator may have been decided by convicted felons who voted illegally in Minnesota's Twin Cities.

That's the finding of an 18-month study conducted by Minnesota Majority, a conservative watchdog group, which found that at least 341 convicted felons in largely Democratic Minneapolis-St. Paul voted illegally in the 2008 Senate race between Franken, a Democrat, and his Republican opponent, then-incumbent Sen. Norm Coleman.

The final recount vote in the race, determined six months after Election Day, showed Franken beat Coleman by 312 votes -- fewer votes than the number of felons whose illegal ballots were counted, according to Minnesota Majority's newly released study, which matched publicly available conviction lists with voting records.

Furthermore, the report charges that efforts to get state and federal authorities to act on its findings have been "stonewalled."

"We aren't trying to change the result of the last election. That legally can't be done," said Dan McGrath, Minnesota Majority's executive director. "We are just trying to make sure the integrity of the next election isn't compromised."

He said his group was largely ignored when it turned over a list of hundreds of names to prosecutors in two of the state's largest counties, Ramsey and Hennepin, where fraud seemed to be the greatest.

A spokesman for both county attorneys' offices belittled the information, saying it was "just plain wrong" and full of errors, which prompted the group to go back and start an in-depth look at the records.

"What we did this time is irrefutable," McGrath said. "We took the voting lists and matched them with conviction lists and then went back to the records and found the roster lists, where voters sign in before walking to the voting booth, and matched them by hand.

"The only way we can be wrong is if someone with the same first, middle and last names, same year of birth as the felon, and living in the same community, has voted. And that isn't very likely."

The report said that in Hennepin County, which in includes Minneapolis, 899 suspected felons had been matched on the county's voting records, and the review showed 289 voters were conclusively matched to felon records. The report says only three people in the county have been charged with voter fraud so far.

A representative of the Hennepin County attorney's office, who declined to give her name, said "there was no one in the office today to talk about the charges."

But the report got a far different review in Ramsey County, which contains St. Paul. Phil Carruthers of the Ramsey County attorney's office said his agency had taken the charges "very seriously" and found that the Minnesota Majority "had done a good job in their review."

The report says that in Ramsey, 460 names on voting records were matched with felon lists, and a further review found 52 were conclusive matches.

Carruthers attributed differences in the numbers to Minnesota Majority's lack of access to nonpublic information, such as exact birth dates and other court records. For example, he said, "public records might show a felon was given 10 years probation, but internal records the county attorney has might show that the probation period was cut to five and the felon was eligible to vote."

Carruthers said Ramsey County is still investigating all the names and has asked that more investigators be hired to complete the process. "So far we have charged 28 people with felonies, have 17 more under review and have 182 cases still open," he said. "And there is a good chance we may match or even exceed their numbers."

McGrath says the report shows that more still has to be done.

"Prosecutors have to act more swiftly in prosecuting cases from the 2008 election to deter fraud in the future," he said, "and the state has to make sure that existing system, that flags convicted felons so voting officials can challenge them at the ballot, is effective. In 90 percent of the cases we looked at, the felons weren't flagged."

"If the state had done that," he said, "things might be very different today."