A US research centre said it found "no evidence of fraud" in Bolivia's presidential elections last October, which was won by incumbent President Evo Morales but had its results dismissed after the Organization of American States (OAS) accused his government of manipulating the results. Morales stepped down in the ensuing uproar.

However, a new study published by researchers at the Massachusetts Institute of Technology's Election Data and Science Lab - commissioned by the Center for Economic and Policy Research (CEPR) - concluded it was "very likely" that Morales' victory was legitimate.

"The media has largely reported the allegations of fraud as fact... However, as specialists in election integrity, we find that the statistical evidence does not support the claim of fraud in Bolivia's October election," the authors wrote in an article published by The Washington Post on Thursday.

On election night, with 83 percent of the votes tallied, official results showed Morales with a seven percentage point lead over his rival Carlos Mesa.

Publication of results was abruptly halted and, when it resumed a day later, 95 percent of the votes had been counted and Morales had the 10-point lead needed to avoid a risky runoff.

Morales said the lead was thanks to votes that came in late from rural Indigenous areas but the OAS mission in Bolivia cast doubt on his explanation, saying the sudden change in voting patterns was "difficult to justify".

On Thursday, in a press release about the study, CEPR said: "Morales appear[ed] to have been heading towards a first-round victory prior to the interruption of the preliminary count.

"The results once the count resumed [were] in line with the prior trend, there was no inexplicable change in trend in the preliminary count as the OAS had claimed," it said.

A prior report also found the rise in Morales's lead was "steady, not drastic.

"The OAS needs to explain why it made these statements and why anyone should trust it when it comes to elections," CEPR's co-director Mark Weisbrot said.

Morales also responded to the report, saying the research provided "further evidence of the monumental theft that .. [was] made to all Bolivians".

"The truth is making its way. The OAS, [Luis] Almagro, and the commission responsible for auditing the results of the October elections, owe many explanations to the Bolivian people and the entire world," he said on Twitter. Almagro is the head of the OAS.

In December, researchers from CEPR joined by 116 economists signed a letter calling on OAS to "retract its misleading statements about the election, which have contributed to the political conflict".

Controversial government

Turmoil in Bolivia began when Morales won a fourth term in office - which was beyond the legal limit - in October and faced immediate resistance from opposition parties that challenged the presidential election results.

Protesters took to the streets, claiming the ballot was rigged.

After weeks of upheaval, Morales resigned under pressure from the military and moved to Mexico, where he was offered political asylum. He was then granted asylum in Argentina.

The country has since been under the stewardship of a controversial interim government led by former Senator Jeanine Anez.

Bolivians will elect their next president, vice president, and other legislators on May 3 after months of political turmoil.