The American Institute of Architects, in a controversial ruling, has decided to suspend an architect from membership for allegedly engaging in unprofessional conduct during construction of the National Visitor Center at Union Station.

Aram H. Mardirosian, a former architectural adviser to the National Park Service, immediately sued the professional society in U.S. District Court here, charging that its standards of ethical practice violate federal antitrust laws.

The AIA imposed a one-year suspension and censured Mardirosian for allegedly "flagrant" violations of two of its ethical standards. he was accused of taking over work for which another architect. Seymour Auerbach, previously had been awarded a contract. Mardirosian was also alleged to have used Auerbach's design work without giving Auerbach credit for its use.

Mardirosian has termed these accusations erroneous and has allegedly in his court suit that the AIA's ethical standards serve illegally to restrain competition among architects. Lawyers for the AIA have declined to comment on Mardirosian's charges. Auerbach has also declined to comment.

In another recent development, the Justice Department and the FBI have closed a criminal investigation of alleged wrongdoing by Mardirosian in connection with the Visitor Center project and a government museum in St. Louis. The Justice Department has said it does not intend to prosecute Mardirosian.

The federal investigation, government records show, focused primarily on events surrounding a National Park Service decision in 1970 to award a more than $200,000 contract to Mardirosian for designing the Museum of Westward Expansion, located under the Gateway Arch in St. Louis.

Mardirosian's architectural firm, The Potomac Group, received the contract only 16 days after Mardirosian left his National Park Service job. The Justice Department and FBI investigated whether a conflict of interest or a conspiracy to violate conflict-of-interest laws had occurred, government records show.

The federal investigation was closed after Craig M. Bradley, a Justice Department lawyer, recommended in a memo that Mardirosian not be prosecuted. Bradley concluded his May 11 memo by saying:

"Thus, though it would appear that Mardirosian did use his position at NPS (the National Park Service) to obtain contracts for himself, a technical conflict of interest. I do not believe that there is sufficient evidence of criminal intent to render this a prosecutable case."

Bradley noted that Mardirosian had apparently been told by a Park Service official at the time that his actions were permissible under ethics regulations.

In an interview, Mardirosian's lawyer, Edward Greensfelder Jr., disputed Bradley's assertion that a "technical conflict existed, saying. "What Mardirosian did not constitute a conflict of interest, technically or subsantively." The disagreement between the Justice Department's and Mardirosian's lawyer over whether a "technical" conflict occurred has not been resolved.

Mardirosian has been embroiled for more than a year in a controversy over contracts he received since 1970 for the Visitor Center and St. Louis projects. According to government records. Mardirosian's firm has been paid almost $1 million for work on both undertakings.Part of this money, according to Mardirosian and his lawyer, was paid to his subcontractors. Mardirosian was an architectural adviser to the National Park Service - a position he had described as part-time and temporary - in 1969 and 1970.

Much of the controversy surrounding Mardirosian was brought into the open by an article in The Washington Post in June, 1976, which reported that the AIA was reviewing Auerbach's charges against Mardirosian. The article also raised questions about Mardirosian's St. Louis contracts.

According to government records. The Post's story immediately led to an investigation by the Interior Department and later to an investigation by the Justice Department and the FBI.

Mardirosian recently obtained files from the three federal agencies of records compiled during their investigation of his activities. The files were obtained through Freedom of Information Act requests. Mardirosian made the files available to The Washington Post this month, saying that he had always acted openly and engaged in no wrongdoing. "I think the record shows that," he added in an interview.

During the recent controversy, Mardirosian's competence as an architect has not been at issue. In addition, Mardirosian has been sharply critical of cost overruns in the construction of the financially troubled National Visitor Center and the still-incomplete parking garage at Union Station. He has repeatedly urged a thorough investigation of how the project was carried out.

The AIA's disciplinary proceedings stemmed from a bitter dispute between Mardirosian and Auerbach, both of whom have been paid hundreds of thousands of dollars for work on Union Station projects. Auerbach first brought charges against Mardirosian before the AIA on May 2, 1975. Though the dates are in dispute. Auerbach's contracts at Union Station were apparently terminated at about this time and Mardirosian replaced him.

Some parts of the AIA's findings were disclosed in Mardirosian's court suit, filed Friday afternoon. In addition, a "confidential" report of the AIA's findings was mailed anonymously to The Post. Its authenticity was confirmed by two lawyers familiar with the case. A three-member AIA panel initially recommended Mardirosian's expulsion from the professional society, but its executive committee later reduced the penalty to suspension and censure, effective July 31.

The AIA panel, these documents show, found that Mardirosian negotiated for and began work at the National Visitor Center while Auerbach's contract for the same work was still in effect. These alleged actions were found to have violated an AIA rule prohibiting an architect from seeking or accepting a job when another architect has an agreement to do the same work.

Mardirosian, disputing these findings, has contended that Auerbach's work at Union Station had been completed before the government began negotiations with Mardinosian to take over the job. In addition, Mardirosian argues in his court suit that the AIA's ethical standard prevents competition among architects in violation of antitrust laws.

Mardirosian's lawyers were joined in the suit by Alan B. Morrison, director of consumer advocate Ralph Nader's Public Citizens Litigation Group. Mardiroisan is seeking reinstatement by the AIA, an injunction barring enforcement of its disputed ethical standard, and damages.

The Justice Department opened its conflict-of-interest investigation of Mardirosian after the Interior Department provided documents that raised questions about the award of the St. Louis museum contract.

Government records show that Mardirosian had discussed the possibility that the contract might be awarded to his firm in conversations with other Park Service officials while he was still a Park Service employee. Mardirosian also drafted a memo, the government records show, that appeared to recommend his own firm for the contract.

Mardirosian and his lawyer have contended that the memo was not intended as a recommendation, but only as a summary of his conversations with other Park Service officials. They have argued that none of these actions violates conflict-of-interest laws or indicate wrongdoing.

Bradley, the Justice Department lawyer in charge of the investigation, recommended against prosecuting mardirosian in his May 11 memo for a number of reasons.

One factor was the statement by a former Park Service personnel office in an FBI interview that he had advised Mardirosian that his actions would not constitute a conflict of interest. Bradley said this alleged advice would provide Mardirosian with "a very significant defense as to criminal intent" if an indictment were sought.

Another factor, Bradley said in his memo, was that "no evidence has been developed of Mardirosian having paid any kickbacks to those responsible for securing this (St. Louis) contract for him." The FBI, according to government records of the investigation, repeatedly sought information about whether kickbacks had been paid, but never turned up any evidence that they had.