The Justice Department, the FBI and the Food and Drug Administration yesterday announced a nationwide crackdown on the $100 million a year black market in illegal steroid drugs, which are believed to be widely used by amateur and professional athletes to build muscles and increase strength.

Eugene Thirolf, the Justice Department attorney who supervised the year-long investigation, said the probe has resulted in the seizure of $2 million worth of prescription steroids in 13 searches in six states. The indictments of three individuals were announced yesterday, and Thirolf said five persons and one corporation have already pleaded guilty in steroid cases. He added that the investigation will continue.

He said illegal steroid usage has been associated mainly with "power lifting, weight lifting, body building, football and the whole range of strength-related sports."

Drug experts said yesterday that they are pushing for increased enforcement efforts because of the extremely dangerous side effects of the steroids, which are synthetic derivatives of the male hormone testosterone.

Among the most serious risks of long-term usage are prostate cancer in men and liver cancer in men and women. The drugs have also been associated with heart disease, strokes, personality changes and reproductive problems, including sterility and birth defects, and the production in women of irreversible male characteristics, such as beards, baldness and deep voices.

Richard Willard, head of the Civil Division at Justice, said medical experts are especially concerned about steroid use by high school athletes because the drugs stop bone growth and can lead to a permanent stunting.

Dr. Gloria Troendle, a senior medical officer with the FDA, said the use of steroid drugs without medical supervision "should be considered life-threatening." She said liver cancer has been found in athletes with as little as one or two years of exposure.

Although athletes claim that steroids improve their strength and athletic performance, Troendle said this has never been substantiated by medical testing because "it is not considered ethical to administer drugs at the level at which they use them ."

Medical experts have estimated that 3 million Americans use illegal steroids. Troendle recited estimates that 80 percent of weight lifters and body builders use them.

The use of steroids has led to scandals in college and professional sports. In late 1983, after 16 athletes, including two Americans, were banned from the Pan American Games after they tested positively for steroids, the National Football League warned its players that nonmedical use of steroids would lead to disciplinary action.

Willard would not say whether the department will focus on the steroid users: "We're not waiving the law for anyone, butthe strategy has been to have maximum impact by targeting traffickers ."

The FDA has issued a "drug alert" bulletin to the 75 pharmaceutical companies and distributors that handle the drug, asking them to make sure their customers are licensed and to stop shipments to any who aren't.

Daniel Michels, chief of compliance for the FDA's Center for Drugs and Biologics, said there will be spot checks to ensure companies are monitoring their customers carefully.

Willard said illegal steroids have so far been seized in in California, New Jersey, Texas, Florida, Ohio and Michigan.

On Wednesday a federal grand jury in Dallas accused Cecil C. Kennedy Jr. and James R. Haga Jr. in an 18-count indictment of conspiring to obtain and sell steroids with a wholesale value of more than $1 million.

In addition, James Bradshaw of Los Angeles was indicted May 13 by a federal grand jury in Fort Lauderdale, Fla., on 23 counts of dispensing animal and human steroids without a prescription. Each count carries a maximum $250,000 fine and three years in prison on conviction.