The House International Relations Committee yesterday launched an investigation into "all aspects" of the murders and suicide at Jonestown, Guyana, including the State Department's handling of complaints about the U.S. religious community there.

Chairman Clement J. Zablocki (D-Wis.) pledged, in effect, to complete the investigation begun two years ago by committee member Leo J. Ryan (D-Calif.), who was murdered by members of the Peoples Temple Saturday at the close of a visit to Jonestown, the cult's commune in Guyana.

Zablocki said the investigation, to be conducted by the committee staff until the 96th Congress convenes in January, will review complaints that the American embassy in Georgetown responded inadequately to complaints from relatives of temple members about forced labor and torture at the four-year-old settlement.

But Zablocki said that inquiry will be just one facet of the committee's probe. "This investigation is not intended to point fingers," he said. "We want to review all aspects of this, from the time the camp down there was started.

State Department officials yesterday said U.S. embassy officers in Guyana had made several trips to Jonestown this year to look into charges of abuse but found no evidence to substantiate the reports.

Following Ryan's murder and the murder or suicide of more than 400 of the cult's followers Saturday, relatives of cultists and former members have portrayed Jonestown as an armed camp in which residents were threatened with death if they resisted the harsh regimen decreed by founder Jim Jones.

The FBI said it will undertake a separate investigation of Ryan's murder, including the possibility of a conspiracy, under a statute that makes killing a member of Congress a federal offense.

The FBI's San Francisco office said it is investigating rumors that Peoples Temple members planned to kidnap or kill other federal officials.

A new mystery surrounding the Rev. Jones' church surfaced Monday when the Guyanese government made public excerpts of letters from 39 prominent Americans praising Jones and the temple. The Guyanese government said it had relied on these "letters of reference" when it agreed to let Jones start his 4,000-acre settlement.

The excerpts referred to letters from, among others, Vice President Mondale; Health, Education and Welfare Secretary Joseph A. Califano, and Sens. Warren G. Magnuson (D-Wash), Henry M. Jackson (D-Wash.) and Mike Gravel (D-Alaska).

None of the five had any record or recollection of such correspondence when contacted yesterday.

A half-dozen other political figures picked at random from the list of 39 names said their flies showed no correspondence to or about Jones. One person on the list who had written Jones was Rosalynn Carter. In 1977, she sent a polite, noncommital reply to a letter Jones had sent her.

The Guyanese embassy here said it does not have the actual letters from which the list of references was compiled, and thus cannot judge their authenticity.

The Social Security Administration said yesterday it had investigated Jonestown early this year after receiving reports that recipients there were assigning their Social Security pensions to Peoples Temple.

That investigation found no evidence of a formal contract assigning pension benefits to the church, a spokesman said, and the probe was terminated. Social Security recpents can hand over each check to others if they wish, but cannot make a formal assignment of their penson benefits.

The Defense Department yesterday dispatched three large HH53 helicopters on the 20-hour flight from Elgin Air Force Base in Florida workers at the settlement.

The military contngent was given a dual mission - finding and treating survivors and identifying and interring the dead.