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A 70-year-old doctor who broke the Los Angeles marathon record for his age group but was disqualified for cheating has been found dead, reports said.

Frank Meza was found in shallow water in the Los Angeles River on Thursday, local station ABC13 reported.

Meza completed the LA marathon in March in 2:53:10, over an hour ahead of the second age group finisher in 4:10:07.

He strenuously denied cheating but was disqualified after organisers said footage showed him leaving the course.

"The video evidence is confirmed by a credible eyewitness report and our calculation that Dr Meza's actual running time for at least one 5K course segment would have had to have been faster than the current 70-74 age-group 5K world record," the organisers said in a statement.

Meza told a Canadian Running magazine that he had stepped off the course only to urinate, arguing that he "didn't know this was against the rules".

Los Angeles police do not believe Meza drowned, according to multiple news reports, but the exact cause of death was not clear.

The retired doctor had been dogged by accusations of cheating before he lined up to run in LA in March.

In 2014, after he completed the California International Marathon in 2:52:33, officials said he had missed several timing mats along the course and banned him from the event.

He drew suspicion after finishing the 2015 LA Marathon in 2:52:47. Organisers said he would need to run the following year's event with an official observer but he declined to take part.

Then earlier this month, Derek Murphy, who runs a website dedicated to catching marathon cheaters, which he said showed Meza riding a bike during part of the 2014 San Francisco Marathon - a race Meza completed in under three hours, winning his age group.

His wife Tina told The Daily Beast that he had been devastated by the cheating allegations.

"Running was very important to my husband. He had been running marathons for the last 30 or 40 years. He was very fast... and now unfortunately he won't run marathons anymore," she said.

She said that he told her on Thursday morning that he was leaving the house for a run, saying, "I'll see you later."