A federal grand jury has indicted four people on conspiracy and fraud charges in an alleged Washington-area charity scheme that prosecutors say netted more than $30,000 ostensibly to aid families of police or firemen disabled or killed on duty.

The 15-count indictment was made public yesterday and charged that the four solicited contributions from nearly 500 businessmen and individuals, providing decals in some cases reading "We Support the Maryland State Police."

Arrested on Monday by the FBI were Michael R. Hendrickson, 31, of Springfield; Lisa Simpson, 27, of Falls Church, and Keith M. Younger, 31, of Fairfax County. The fourth defendant, John I. Salemi, 29, address unknown, was being sought yesterday. Hendrickson, Simpson and Younger were later released on $5,000 bond.

Prosecutors said the four claimed to represent the Maryland Fraternal Order of Police or Police and Firemen's American Legion Post 29, a now-inactive chapter in the District.

Between May 1979 and February 1980, the indictment said, donors were solicited by telephone and asked to write checks to buy advertising for a nonexistent yearbook to benefit the widows and orphans of police or firemen. The money was divided among the four defendants using two accounts at Diplomat National Bank in the District, prosecutors charged.

According to the indictment, the defendants opened a bank account on May 20, 1979, at Diplomat under the name Fraternity of Painters and deposited checks in it made out to FOP. A second account in the name of the American Legion post was opened on Feb. 26, 1980.

"A lot of people gave two or three times a year, usually less than $100," Assistant U.S. Attorney Thomas Buchanan said yesterday. "Most people wanted to make a charitable contribution. They didn't care about the ads and they didn't ask any questions."

Buchanan said Hendrickson and an unidentified, unindicted coconspirator who aided prosecutors in the case had worked in the past for John J. Gray, a professional publisher and fund-raiser in the District who Buchanan said is "very well thought of" in the field. The defendants "all have a lot of experience and a background in legitimate telephone solicitation," the prosecutor said.

James Smith, a retired D.C. police lieutenant and past commander of Legion Post 29, said yesterday that a magazine published by Gray for members of the post had been terminated about four years ago, shortly before the post itself went out of business because of declining membership.

Gray could not be reached for comment yesterday.

Attorney C. Philip Nichols of Laurel, who represents the Maryland State Fraternal Order of Police, said yesterday he was unaware of the alleged fraud. "We're appalled by the use of the organization's good name," Nichols said.

If convicted, the four defendants could receive up to five years in prison and a $10,000 fine on the conspiracy count. Each fraud count is punishable by five years imprisonment and a $1,000 fine.