New York ? Top Republicans on Wednesday tried to tie the Kerry campaign to disputed documents used by CBS News for a story examining President Bush’s Vietnam-era service in the Texas National Guard and called for a congressional investigation.

Meanwhile, CBS News President Andrew Heyward promised to work harder to answer questions about the veracity of memos it believes were written by the president’s late National Guard commander.

The network continues to maintain that “the content” of the story is true.

The documents written by Lt. Col. Jerry B. Killian indicated he was being pressured to sugarcoat the performance ratings of a young Bush, then the son of a Texas congressman, and that Bush failed to follow orders to take a physical. Several experts believe they are fakes, prepared on a computer about events in a typewriter era.

On the “CBS Evening News” Wednesday, CBS interviewed Killian’s former secretary, who believes the documents are fake but they accurately reflected her former boss’ feelings.

“I know that I didn’t type them,” said 86-year-old Marian Carr Knox. “However, the information in those is correct.”

In calling for a congressional investigation, California Rep. Chris Cox said the network has declined to reveal its source “despite the growing abundance of evidence that CBS News has aided and abetted fraud.”

The chairman of the House Committee on Energy and Commerce, fellow Republican Joe Barton of Texas, rejected Cox’s call for a hearing.

“A news organization’s responsibility is to facts and truth, but the oversight of network news generally is a matter best sorted out by the viewing public and the news media,” Barton said. “I do not personally believe these documents are legitimate, and it seems clear that the press and the two presidential campaigns are properly dealing with that issue.”

Forty congressional Republicans joined House Majority Whip Roy Blunt, R-Missouri, in a letter calling on Heyward to retract the story.

“To date, CBS’s response to the specific and devastating criticisms of the accuracy of its reporting has been to question the motives of its critics, to offer half-truths in its own defense, to refuse to disclose crucial evidence and to circle the wagons,” Blunt wrote.

The Republican National Committee cited CBS in criticizing a Democratic National Committee video that portrays Bush as a “fortunate son” who used family connections to dodge the Vietnam War and then lied about it.

RNC spokesman Jim Dyke said the video was “as creative and accurate as the memos they gave CBS.”

Responded Phil Singer, Kerry campaign spokesman: “It’s ridiculous. We didn’t give CBS anything.”

CBS did not respond to the political accusations. Heyward said that “we would not have put the report on the air if we did not believe in every aspect of it.”

However, he added, “enough questions have been raised that we are going to redouble our effort to answer those questions.”

CBS has refused to reveal how it obtained the documents, other than it received them from a man who “had access to the documents he provided and an opportunity to obtain copies of them. Our sources included individuals who had first-hand knowledge of the events in question.”