The FBI is investigating whether Yugoslav secret agents are waging a campaign of extortion and violence against ethnic Croatians in Los Angeles and other major U.S. cities.

At the same time, the bureau has publicly denied rumors, widely circulating among Croation separatist groups, of FBI collusion with Yugoslavian President Josip Tito's secret police.

the Extraordinary gesture came in the form of a four-page statement issued Thursday by Herbert D. Clough Jr., special agent in charge of the FBI's Los Angeles office and former chief of counterintelligence operations here.

Since last summer, when extortion letters were mailed to about 50 prominent Croatians from a self-styled separatist group claiming to be based in Paraguay, there has been a rash of bombings and shootings directed against those who balked at paying from $5,000 to $15,000.

One letter recipient, Kris Brkic, president of a coalition of 19 Croation community organizations, was shot to death Nov. 22 on the lawn of his home in Glendale.

On April 6, bombs exploded at the Rossmoor home of Mario Forgiarini, Brkic's successor, and at the Cerritos residence of Frank Striskovich, another Croation community activist.

On May 23, Croatian nationals Zvonko Simac, 25, and Mario Rudela, 21, were killed when a pipe bomb they were transporting accidentally exploded in their pickup truck in San Pedro.

In his statement, Clough said the FBI and other law enforcement agencies have been "hampered by the unwillingness of many Croations to provide information" about the case.

There are about 100,000 people of Croation descent living in the Los Angeles metropolitan area, many of them in and around San Pedro.

Their unwillingness to cooperate with investigators, said Clough, "apparently stems from the widespread belief among Croatians that any acts of violence committed against Croations are the work of the Yugoslav secret police, commonly referred to as UBDA, and many Croatians apparently believe the FBI routinely provides information to UDBA."

Characterizing that claim as "absolutely not true," Clough declared that the FBI maintains no contacts with UDBA nor has it furnished any information to it.

On the other hand, Clough lent some credence to another suspicion in the Croatian community - that Tito's agents have infiltrated Croatian separatist groups and are sowing the seeds of internecine conflict to render those organizations impotent.

Clough revealed that the investigation is centering on "radical elements" within a Croatian organization known as OTPOR, a Croatian term for resistance. The two men killed in the San Pedro blast were believed to have been members of the organization.

Noting that investigators were hard put to explain why the violence was directed against Croatians by Croatians, the FBI official added: "A possible explaination lies in the widespread belief among Croatians that OTPOR has been infiltrated by Yugoslav agents who are directing the violence . . ."

In an interview with the Los Angeles Times, Clough declined to elaborate on his reference to possible infiltration by Yugoslav agents and refused to say whether the FBI had indenpendently established any links between Tito's secret police and the recent violence.

"But," he added, "we are not ignoring that or any other possiblity."

An official at the Yugoslav embassy in Washington denied that UDBA agents were operating in this country.