More than 50 Northern Virginia residents, including nine teen-agers, have been indicted in Loudoun County's biggest recent crackdown on drug trafficking, authorities said yesterday.

The indictments, ordered sealed by a circuit court judge in Leesburg, accuse 53 Loudoun, Fairfax and Prince William county residents of selling and possessing cocaine, marijuana and hallucinogenic drugs, including LSD and PCP, a spokesman for the Loudoun sheriff's office said. Thirty-eight suspects were arrested Thursday night and others were expected to be arrested shortly, the spokesman said.

During a five-month investigation, undercover agents bought $54,000 worth of marijuana, $16,000 of PCP and $700 of cocaine at a business establishment and a high school in the eastern part of the county, according to Capt. Leonard McDonald, the sheriff's spokesman. McDonald said investigators turned down opportunities to buy larger amounts of cocaine because of insufficient funds. Several suspects were accused of supplying LSD to high school students, he said.

"The investigations are still continuing into other areas," McDonald said. The indictments were returned Tuesday by a grand jury, he said, and were ordered sealed by Judge Carlton Penn until April 6, when arraignments are scheduled. McDonald declined to identify the school or the business establishment where drug sales allegedly occurred.