WASHINGTON (AP) _The FBI assigned a “secret” classification to material circulated publicly by a conservative group and sent it to more than 30 field offices in an investigation of opponents of the Reagan administration’s Central American policies, according to an FBI document and interviews.

The conservative group aide who compiled the information said Thursday he found it “somewhat humorous” that the material was classified and “became part of a major investigation.”

The aide, Mike Boos of the Young Africa’s Foundation of Reston, Va., said he sent the same material to about 500 conservative individuals and organizations in the Washington area and published it in a conservative newsletter, The American Sentinel.

FBI spokesman Ray McElhaney said the material “contained allegations of criminal wrongdoing over which the FBI has criminal jurisdiction,” and the bureau was “duty bound to check it out.”

McElhaney said he did not know why the public material was classified when it was received in 1984, even though Boos told the bureau in his cover letter that it would be published in the newsletter.

The July 12, 1984, FBI letter to 33 field offices, which accompanied Boos’ material, was declassified last September and released recently to the New York-based Center for Constitutional Rights, which filed a Freedom of Information Act lawsuit to obtain the material.

The center was representing the Committee in Solidarity With the People of El Salvador—CISPES—which was the focal point of the FBI investigation.

The center said the surveillance of CISPES, which began in 1981, branched out to include other organizations opposed to U.S. policies in Central America.

The bureau said its full-blown investigation began March 30, 1983, and ended June 18, 1985. McElhaney said the FBI inquiry was conducted under guidelines on international terrorism investigations, most of which are classified. No charges were filed.

He said the investigation was “predicated upon information from several sources that certain key members of CISES were involved in overtly furnishing funds and materials to a foreign terrorist organization”—the Frente Farabundo Marti Para La Liberacion Nacional—(FMLN).

CISPES spokeswoman Beth Perry said Thursday, “The entire FBI investigation, which took five years, went on too long to prove there was no criminal or terrorist activity.

Under Attorney General Guidelines for FBI Foreign Intelligence Collection and Foreign Counterintelligence Investigations, the bureau can investigate activities conducted “for or on behalf of a foreign power.”

Targets can include “an individual or organization that is the object of a recruitment effort by a foreign intelligence service or international terrorists; or information, persons, property or activities in the United States that are the object of intelligence activity by a foreign intelligence service or international terrorists.”

Boos said in an interview that in 1982, CISPES members had disrupted a campus speech arranged by his group in Washington.

Boos said he, in turn, monitored CISPES and on June 9, 1984, attended a public meeting of CISPES‘ chapter in the nation’s capital.

On June 25, 1984, he sent the FBI materials obtained at the meeting and the article he wrote for The American Sentinel.