Congressional investigators examining allegations of taxpayer and employee abuse at the Internal Revenue Service were unable to substantiate the claims, made last year during a Senate hearing, because the IRS kept incomplete records or did not collect data on the cases, the General Accounting Office said in a report issued yesterday.

Last year's hearings, combined with allegations leveled in 1997 hearings, helped spur passage of legislation overhauling IRS management and agency procedures. But various internal probes have failed to show that the abuses highlighted in the hearings were widespread.

IRS Commissioner Charles O. Rossotti, in a written statement yesterday, signaled that he believes "we have learned all we can about these particular cases" and suggested the IRS needs to get on with "the broader issue of modernizing the IRS to serve taxpayers."

Rossotti said the IRS has adopted new methods to ensure employees can be held accountable for their actions and has crafted "balanced measures" aimed at improving the agency's performance.

"We certainly cannot undo what happened in the past, but we can take concrete actions today and in the future to ensure that it is not repeated," Rossotti said.

But Sen. William V. Roth Jr. (R-Del.), who chaired the Senate Finance Committee hearings on the IRS and asked for the GAO probe, said the report "raises issues that give us serious concern."

In a written statement, Roth said the GAO report "makes it clear that satisfactory results cannot be achieved until the IRS systematically tracks relevant data and keeps information necessary for real reform."

Witnesses at Roth's hearings alleged that senior IRS managers were not subject to the same level of disciplinary action as line staff, that the agency retaliated against whistle-blowers and against taxpayers who were perceived as noncooperative, and that IRS employees eliminated or reduced proposed tax assessments for reasons not related to the merits of individual cases.

But inadequate IRS records and a lack of data prevented the GAO from drawing conclusions about the allegations, the report said. Also, the GAO noted that some of the allegations involved taxpayer data that are protected by privacy laws.