MAROLYN THOMPSON of Fremont, Calif., came to Washington this week to tell Congress how her daughter was abducted and kept for seven months last year by her father, Thompson's former husband. The man, she told a House judiciary subcommittee, had a history of alcoholism, mental problems and drug use. She spent $28,000 in detectives, lawyers, trips and phone bills in a search that led to a rundown motel room where the child had been living a few feet from the swimming pool. The child was 2 year old.

Sandra Coleman of Myrtle Beach, S.C., also came to town to tell Congress her story, only it doesn't have such a happy ending. Her son, an only child, was 6 years old when he was snatched by his father. The child suffered from severe allergies for which Coleman, a nurse, had been giving him shots. The father knew nothing of the medical treatment when he took the child three years ago. Mrs. Coleman has spent $30,000 in detective fees, phone bills, legal and travel expenses to track down deadend tips. The child and the father have never been found.

Child snatching has reached epidemic proportions in the United States, according to the witnesses, with anywhere from 25,000 to 100,000 children being snatched from one parent by another each year. Since mothers are still the ones usually given custody, most child snatchers are fathers.

"Out of every 10 snatchings, six or seven out of the 10 are never again seen by the parent left behind," testified Doris Jonas Freed, a member of the council of the American Bar Association's family law section. Even though the parent doing the snatching may think he is a loving parent, "when the child is recovered he has suffered ineradicable trauma," leading to nightmares and years of psychiatric help, she said. "The objective results to the child are really appalling, and come within the definition of child abuse." t

State laws have worked to the benefit of the parent who snatches his child and then shows up with the child in another state's court seeking custody. In the past, many states did not have laws punishing child snatchers and "all you had to do was if you lose in one state, go to another state and hope for another decision," says Freed.

But in the last several years, some 44 states have enacted the Uniform Child Custody Jurisdiction Act, a reciprocal act in which the states agree to honor out-of-state custody awards. This significantly reduces the chances of a parent going to another state and getting a new or modified decree.

(The District of Columbia is among the jurisdictions that have not enacted the law and remain, at least theoretically, havens for child snatchers, unless they have fled a state in which child snatching is a felony. In that case, the state can extradite the child snatcher from the city.)

Legislation is now being considered by Congress that would extend the act to all the states and that would, in addition, enable law enforcement officials to use the federal Parent Locator Service to track down snatching parents through their jobs. That service is currently used to find spouses who are delinquent in their child support payments. Congress is also considering legislation that would make it a federal misdemeanor for a parent to snatch a child, a law which would allow the FBI to get involved in these cases.

The FBI now claims it does not have the resources to get involved in child snatching cases, which it considers to be a domestic matter and out of its jurisdiction. Marolyn Thompson was able to get the FBI involved because child snatching is a felony in California. A felony warrant was issued for the father, she could prove the father had crossed state lines with the child and that the child was in danger. Mrs. Coleman, in spite of her son's allergies, was told by the FBI that it couldn't get involved. "He could die because of this, yet the FBI wouldn't touch it. I don't know what they consider life-endangering," she said.

An FBI official told the committee the bureau is currently is handling only 16 cases of child snatching, and that it is limiting wiretaps and mail covers -- which might disclose the location of the missing parent when he contacts relatives -- to such major cases as those involving national security. "Even when the father is a drug addict or a psycho, God forbid you should use a mail cover or a wiretap," Rep. Henry Hyde, (R-Ill.) observed acidly.

No one is suggesting that the FBI go after 25,000 parents a year. Hyde, for example, said local magistrates could screen the most egregious cases for referral to the FBI. And Freed pointed out that the simple threat of FBI involvement would deter parents from snatching their children.

Child snatching, unheard of a few years ago, has rapidly become one of the most devastating phenomenon in the American way of divorce. While more and more states are making the crime a felony and refusing to modify another state's custody decree, it is clear that federal government needs to get interested in the problem.

At the end of her testimony, Sandra Coleman asked, pleadingly, how four people -- her former husband, his new family, and her son -- could simply drop off the face of the earth.

But she already knew the answer. She hasn't seen her only child for three years because not enough of the right people got involved.