A federal appeals court Friday put an end to the Federal Election Commission’s 25-year-old practice of releasing its files from closed investigations of political groups and candidates, ruling that such disclosures violated First Amendment rights.

“We hold that the regulation, though not contrary to the plain language of the statute, is nevertheless impermissible because it fails to account for the substantial First Amendment interests implicated in releasing political groups’ strategic documents and other internal materials,” U.S. Circuit Judge David Tatel wrote.

But Tatel’s opinion, which was joined by U.S. Circuit Judge David Sentelle, held out the possibility that the FEC could craft new disclosure rules that would address First Amendment concerns.

The third judge on the panel of the U.S. Court of Appeals for the D.C. Circuit, Karen LeCraft Henderson, concurred in the result but wrote that she would have struck down the FEC regulation on different grounds.

“It is an unfortunate decision,” said Larry Noble, the former FEC general counsel who now heads the Center for Responsive Politics, a watchdog group.

“For 25 years, the FEC has been releasing closed investigatory files so that the public can see what is happening with these cases, and this now stops that practice and keeps a shroud over over these files,” Noble said. “And it’s unfortunate because with the FEC, there is always a concern about whether the FEC is really enforcing the law.”

Noble noted the FEC enforcement practices are going to play an even more critical role under the new campaign finance law, which substantially increased the penalties for violating election and campaign finance rules.

An FEC spokesman said the agency’s lawyers were still studying the opinion and had no comment on whether it would be appealed.

The case grew out of an FEC investigation of alleged coordinated activity between the AFL-CIO and Democrats in the 1996 election. In 2001, the union objected when the FEC began putting thousands of pages of material it had collected during its aborted investigation on to the public docket.

The agency had obtained more than 50,000 pages of potentially sensitive internal union documents and had briefly made public about 6,000 pages of material that revealed examples of close coordination between Democrats and unions, showing that labor officials often served on party steering committees to help approve or reject Democratic campaign plans.

The AFL-CIO argued that release of the documents would harm organized labor because it would reveal confidential plans and strategies and make it difficult to recruit people to work on the effort. A lower court agreed and blocked the FEC from putting the material on public record.

In its appeal, the FEC argued that its 25-year-old practice was a reasonable interpretation of Congressional intent that allowed the public to evaluate its enforcement record and that no one had ever produced evidence of substantial First Amendment harm from the disclosures.

In his ruling, Tatel noted that while the court would normally defer to the FEC’s rulemaking, the disclosure rule at issue had to be examined more closely on First Amendment grounds. He cited a long line of cases involving government-mandated disclosure requirements that emphasized the potential damage to political groups’ effectiveness, in addition to the risk of chilling individual participation.

Tatel was also persuaded by arguments made by the AFL-CIO and its conservative ally in this case — the James Madison Center — that the FEC’s disclosure practice provided a strong incentive for opposing political groups to file complaints against one another, if for nothing else than the possibility that confidential material would be made public.

But Tatel suggested that the FEC could come up with a new disclosure rule that would account for the First Amendment concerns.

“In sum, although we agree that deterring future violations and promoting Commission accountability may well justify releasing more information … the Commission must attempt to avoid unnecessarily infringing on First Amendment interests where it regularly subpoenas materials of a delicate nature representing the very heart of the organism which the first amendment was intended to nurture and protect,” Tatel wrote.

Because the FEC regulation failed to “undertake this tailoring, it creates the ‘‘serious constitutional difficulties outlined above,” the judge continued.

Larry Gold, the attorney who argued the case on behalf of the AFL-CIO, said he was pleased with the ruling.

“The focus on the First Amendment aspect was very important and correct,” he said. “The court truly appreciated the central point, which is that an agency whose mission is to regulate political activity has to be very careful with the kinds of information that it gathers and reveals.”

In her concurring opinion, Henderson, one of the three judges who issued the bitterly divided opinion on the constitutionality of the new Bipartisan Campaign Reform Act, used some comic relief to make her point that the FEC regulation didn’t comply with the Congressionally written election law statute.

Henderson found that Congress had meant to keep investigative files confidential and was not impressed with FEC arguments that the words of the statute could be interpreted differently.

“Taken to its logical conclusion, the FEC’s argument would render every prohibition in the United States Code susceptible of ambiguity,” she wrote. “‘Thou shall not kill’ is a mandate neither silent nor ambiguous about whether murder is permissible if committed after 5:00 p.m. — or, for that matter, if committed in the billiard room with the candlestick — but the FEC’s reasoning would lead one to conclude otherwise.”