Hyacinthe (Heidi) T. Napper, administrative assistant to Rep. John Conyers Jr. (D-Mich.), surrendered to authorities yesterday after U.S. postal inspectors charged her with passport fraud in connection with a federal investigation of Black Hebrew Israelites, a religious sect to which Napper belongs.

U.S. Postal Inspector T.W. McClure said Napper, 53, of 3520 Pope St. SE, was charged with allegedly "using a fictitious name on a passport and using the phony passport to travel back and forth to Israel." Napper, who is paid $45,000 a year, has worked for the Detroit congressman for 17 years. She could not be reached for comment.

Conyers, reached last night at his home in Detroit, declined comment on the arrest except to say that he was investigating the incident. "I understand from news reports that she has denied it," he added.

Ray Crittenden, a staff assistant in Conyers' Washington office, said last night that Napper's alleged connection to the sect and her surrender yesterday came as a "complete surprise" to her Washington office colleagues. Crittenden, who has worked in the office with Napper for about a year, described her as a "quiet and efficient worker." Crittenden also said that office staffers' only knowledge of the Black Hebrew Israelites sect was a vague recollection of having heard something about it on the "60 Minutes" television show some time ago.

The Black Hebrews, whose organization is formally known as the Original Hebrew Israelite Nation of Jerusalem, have attracted the attention of law enforcement agencies for more than a decade since their emergence in Chicago's south side ghetto during the late 1960s.

The group claims 3,000 members who are scattered throughout the United States, Ghana, Liberia and Israel. In the past year, a half-dozen members have been convicted on a series of charges, such as credit card fraud and international transportation of stolen securities. Another 40 have been indicted.

The organization is led by Ben Ami Carter, a former Chicago truck driver who preaches that all blacks are Jewish descendants of the Biblical patriarchs Abraham, Isaac and Jacob.

The warrant for Napper's arrest was issued Monday in New York, six months after a team of postal inspectors and FBI agents searched a Black Hebrew house in Hyattsville and seized what they alleged was a large cache of stolen mail, forged checks and documents. Among the 57 passports that federal agents confiscated was one that Postal Inspector Thomas R. Denneny said contains a photograph of Napper. However, police said the passport was issued to La Fawn Snow, a 27-year-old female.

Napper is alleged to have used the Snow passport to enter the U.S. at New York on Jan. 3, 1981, when she returned from a trip to Israel. In addition, federal authorities charge that Napper forged a customs declaration on the same day when she allegedly signed Snow's name.

Napper was released on an unsecured bond of $50,000.