The evidence is mounting that Iranian diplomats have blood on their hands in the murder of an Iranian dissident in Switzerland in April, and that Iranian President Hashemi Rafsanjani was personally behind the hit.

The case is receiving special scrutiny by the CIA because it is critical to its assessment of Rafsanjani. Is he a moderate who can clean up Iran, or simply a clone of the Ayatollah Khomeini?

The murder victim was Dr. Kazem Rajavi, a respected political science professor at Geneva University, living in exile from his native Iran. Over the past few years he has been a burr under the saddle of Khomeini and Rafsanjani. From his safe place in Switzerland, he has criticized the bloodshed, terrorism and torture perpetrated by the Iranian government.

Making Rajavi even more unpalatable to Iran is the fact that his brother is the leader of the largest Iranian resistance group, the People's Mojahedin, based in Iraq.

Rajavi was so outspoken that the Iranian envoy to the United Nations in Geneva threatened, in front of witnesses, to send a hit squad to ''liquidate'' him.

On April 22, while the world was thanking Iran for the release of American hostage Robert Polhill, assassins were on their way to do their work. While basking in the front-page coverage of the breakthrough in the hostage dilemma, Iranian officials secretly gave the go-ahead to the hit team to get rid of the irksome Rajavi.

Rajavi had been to the bank on the morning of April 24 and was on his way back home to a suburb of Geneva when two cars blocked the road. Witnesses reported that three gunmen jumped out of the cars and began shooting. After Rajavi fell, one of the gunman shot him in the head. Doctors found 13 bullets in Rajavi's body.

The names and photos of two suspected assassins have been released by Swiss police. The two men stayed at the Langchamp Hotel, where all Iranian consulate guests stay while visiting Geneva.

Sketchy news reports right after the shooting did not say Rajavi was dead, so the assassins had to get word to Iran that their mission was accomplished. We have a copy of the transcript of an intercepted telephone conversation at that time. On the Geneva end was Karim Abadi, the Iranian consul general. On the Tehran end was an intelligence official with Rafsanjani's Revolutionary Guards Corps.

The men talked cryptically at first, alluding to the fate of the assassins without mentioning Kazem Rajavi. The agent in Iran asked if ''both of them'' are ''safe.'' Then he slipped and said, ''I'm talking about Dr. Kazem's case.'' Abadi responds, ''He no longer has any problem at all. His problem is solved.''

The agent in Iran still fretted: ''You know this was the worry of Hajji himself . . .'' Hajji is a reverential term for Rafsanjani.

Then the agent pressed for more details. He asked Abadi if ''their Golf car'' is ''in your possession.'' Abadi said it was. Swiss police say the assassins drove a rented Volkswagen Golf. Police later found it abandoned with a dented front end.

The agent in Tehran then added his regards to ''Mr. Malaek'' and asked if Malaek could help one of the ''fellows'' that Tehran is worried about. Mohammed Hossein Malaek is the Iranian ambassador to Switzerland.

Within hours after the phone call, one or more of the assassins and some diplomats boarded a getaway plane to Tehran. It was the weekly Iran Air flight out of Geneva. Witnesses say it was held up for an hour and then took off after three men with Iranian diplomatic passports rushed to get aboard. The passenger list of the flight included the name of one of the suspected assassins.

This case even has a smoking gun. Swiss authorities allowed one Iranian diplomat, Manouchehr Taleh, the former consul general in Geneva before Abadi, to carry a gun in Geneva. We have a copy of the permit that Taleh used to authorize him to carry a 9mm Beretta pistol. He told Swiss police he needed it because of threats against his life. Some of the bullets in Rajavi's body came from a 9mm pistol. Swiss police are secretly trying to find out whether the bullets came from Taleh's gun.

We have also obtained a copy of a top-secret memo from Taleh dated Dec. 13, 1988. In it he reports to his superiors in Tehran that a Swiss lawyer with good access to Swiss police is willing to sell, for $3,000, the secret police list of the names and addresses of Iranians living in Switzerland. Rajavi's home address was reportedly on that list.