The United Nations whitewash last February of Iran's human rights record was soon followed by the murder in Geneva of a famed anti-Khomeini leader, an event so improbable as pure coincidence that the United States is quietly helping the victim's widow seek retribution from U.N. Secretary General Javier Perez de Cuellar.

Dr. Kazem Rajavi was gunned down at noon April 24 by machine-gun fire from a rented car a few yards from his driveway in a fashionable Geneva suburb. He became the target of the mullah heirs to the Ayatollah Khomeini for two reasons: he was putting out rare knowledge of officially sponsored murder and torture in Iran; his brother, Iraqi-based Massoud Rajavi, leads the People's Mujaheddin resistance.

Hours after the murder, three Iranians with diplomatic passports rushed through the Geneva airport and onto Iran Air's once-a-week Geneva-Tehran nonstop flight. Departure was delayed for 45 minutes awaiting their arrival.

Now, after a two-month police investigation, the Swiss government has just informed Dr. Rajavi's widow, Michelle, that it is having trouble proving Iranian government complicity. To U.S. officials, that smells like a political cop-out: the neutral Swiss avoiding a major ruckus with dangerously unpredictable Iran.

The Swiss are not alone. The U.N. report constituted a corrupt deal by the international community to disregard Iranian human rights atrocities in return for a final settlement of the Iran-Iraq war. Nor are American hands entirely clean.

The Reagan and Bush administrations for several years have rejected demands from Congress that the United States start dealing with the People's Mujaheddin. Nearly 200 House members, including such key figures as Republican Whip Newt Gingrich (Ga.), Democratic Whip William H. Gray (Pa.) and House Foreign Affairs subcommittee chairman Mervin Dymally (D-Calif.), last September sent a letter of complaint to Secretary of State James A. Baker III .

They want the administration to take ''practical steps'' to highlight Iran's ''gross violations'' of human rights and at the same time open talks with the 12 separate resistance groups. The People's Mujaheddin comprise by far the largest of these.

Michelle Rajavi came here from Geneva last week in pursuit of justice for her dead husband. Her first stop on Capitol Hill was Dymally, chairman of the international operations subcommittee. Dymally suspects a ''hidden agenda'' between the U.N. Human Rights Commission in Geneva and the U.N.'s broader efforts to arrange a settlement of the Iran-Iraq war. The commission's special probe in Iran ignored evidence of official complicity in murder and torture as well as Dr. Rajavi's valuable files in Geneva.

What kind of ''hidden agenda''? we asked Dymally. An invaluable political payoff to the Khomeini mullahs, he replied. The U.N. report repudiated everything the slain Rajavi, the regime's dangerous enemy, had been saying from Geneva, and it sent the rest of the world an engraved invitation to open warmer relations with the miraculously human rights-pure mullahs.

''The report was an outrage,'' Dymally told us. As soon as the Human Rights Commission issued it, the regime had what it wanted before murdering Rajavi. ''They delayed the operation until the {human rights} report was released,'' Dymally said.

In an interview, Rajavi's widow gave us a terrifying but credible account of how, both before and after the murder, Iranians had terrorized their 16-year old daughter, poisoned the family's German shepherd and blocked access to the garage, while their agents stood across the street for hours leering at the house. An Iranian ''diplomat'' warned Rajavi at a January meeting of the Human Rights Commission that ''we will kill you."

Two days before his murder, 10 Iranians arrived in Geneva from Tehran, bearing diplomatic passports and the appearance of thugs. ''Everyone knew these were not diplomats,'' Michelle Rajavi told us. On the night before the murder, a van filled with Iranians followed the Rajavis as they drove into Geneva for dinner, weaving around their car. ''They were inspecting my husband,'' she said.

Justice will not come easy. When the Human Rights Commission issued the whitewash report, the United States filed a mild disclaimer. ''Unfortunately, human rights violations are still continuing in Iran,'' the United States said. ''Nothing has changed.''

Ambassador Ronald Spiers, top U.S. diplomat attached to Perez de Cuellar's office, informed Dymally and other angry congressmen last week that he would arrange a meeting between Michelle Rajavi and the secretary general. But that is small potatoes. The real need is for the United States to prevent the Human Rights Commission from undermining the very rights it is committed to uphold. To do so encourages murder.