LAGOS, Nigeria (CNN) -- The former Nigerian opposition leader Moshood Abiola died of natural causes earlier this week, according to Saturday's preliminary report by a team of international pathologists.

"At this time, our preliminary opinion is that death was due to natural causes as a result of his long-standing heart disease," said a statement by the team, which had carried out the autopsy a few hours previous to Abiola's funeral in Lagos on Saturday.

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"After a detailed review of the observations of witnesses present at the time of death, we believe that poisoning is extremely unlikely," the pathologists' statement said.

Further testing was to be accomplished over the next three weeks since tissue samples would be sent to Canada and Britain, the pathologists said.

It was unclear whether the preliminary findings would appease Abiola supporters and some family members who had expressed suspicion about the official death cause. Abiola collapsed during Tuesday's talks with a visiting U.S. delegation to discuss his possible release after years of detention.

The military government said Tuesday Abiola had died of cardiac arrest but his supporters as well as some family members expressed skepticism about that official account.

The Associated Press and Reuters contributed to this report.