An autopsy and tests on Slobodan Milosevic's blood found no evidence of poison or drugs in concentrations that could have killed him, the U.N. war crimes tribunal said Friday.

Tribunal president Judge Fausto Pocar also said an outside investigation will be conducted on the running of the U.N. detention center where Milosevic was held during his four-year trial and where he died March 11.

Milosevic was ruled to have died of a heart attack, but questions were raised by his son and his supporters about the cause of death after it was reported he had been taking medicines that were not prescribed by the U.N. cardiologist.

"No evidence of poisoning has been found," Pocar said, reading a report's preliminary results. A number of prescribed medications were found in his body, "but not in toxic concentrations," he said.

He also said no traces were found of the powerful antibiotic rifampicine, which a Dutch toxicologist had reported finding in a blood sample taken from the Serb leader earlier this year.

Rifampicine, which affects the liver's ability to break down enzymes, was thought to have blunted the affect of the beta-blockers Milosevic was taking for his blood pressure, leading to speculation that it could have contributed to his death.

Since the drug disappears quickly from the body, the report said, it was unlikely that it "had been ingested or administered in the last few days before death."

The toxicologist who submitted a report Feb. 12 on Milosevic, Donald Uges, said he believed the detainee took the antibiotic to show that he was not receiving adequate treatment and to press his demand to the tribunal for temporarily release to go to a Moscow heart clinic. The judges repeatedly denied his request.

Milosevic "received the best possible treatment," Pocar said.

Further tests were being conducted by the Netherlands Forensic Institute which conducted the autopsy March 12, Pocar said, and the conclusions were only provisional.

The initial autopsy report confirmed Milosevic suffered two heart ailments that would explain the heart attack. Tribunal transcripts cite previous medical reports that he had high blood pressure, which caused hypertrophy, or a thickening of the wall, of the left ventricle.

The results of the tests were delivered by the Dutch prosecutor's office to both the tribunal registrar, Hans Holthuis, and to Milosevic's lawyer, Zdenko Tomanovic, Pocar said.

Tomanovic and Milosevic's son, Marko, who came to The Hague to claim the body and sent it to Belgrade for a funeral Saturday, claimed Milosevic had been killed and accused the tribunal of responsibility for his death.

Milosevic's casket is on display in a museum dedicated to the late communist dictator Josip Broz Tito in a Belgrade suburb, where thousands have viewed it. Milosevic will be buried Saturday at a family estate in his hometown of Pozarevac, about 30 miles southeast of the capital.

Holthuis, the tribunal's administrative head, ordered an outside investigation to find out how Milosevic obtained drugs he was not supposed to have.

Tribunal officials earlier said Milosevic, reputedly a heavy drinker and smoker, also had regular access to alcohol and unprescribed medication.

Holthuis said it was "the burning question" how Milosevic obtained illicit goods in prison. He said Milosevic's cell was regularly searched, and when contraband was found, "we acted in a proper way."

Milosevic was accorded special privileges because he was defending himself. He was allowed a special office where he could meet with legal advisers, and he also interviewed more than 70 witnesses he intended to call at his trial. These witnesses were not subjected to the usual rigorous searches when they entered and left the detention unit.

"I have full confidence in the professionalism" of the prison and its staff, which underwent frequent independent inspections, Pocar said.