A Green Beret who died in mysterious circumstances while on a secret assignment in west Africa had reportedly declined to accept illicit money from Navy SEALs before his death.

US Army Staff Sgt Logan Melgar, a 34-year-old veteran of two tours in Afghanistan, was found dead on 4 June in the embassy housing he shared with other special operations members in Bamako, Mali.

In a development that has sent shockwaves through the special operations community, two members of the elite SEAL Team Six have since been accused of strangling their fellow US serviceman.

And five members of the special-operations community have now said Melgar's death came after he had discovered the SEALs were pocketing money intended to pay local informants.

He was offered a cut of the illicit funds by the Navy commandos but he declined, the sources told The Daily Beast.

It was also reported that prior to his death, Melgar had told his wife of problems he had been having with two of the SEALs and that he would elaborate once he was home from his six-month posting.

Melgar, a Texan who served in the US Army's 3rd Special Forces Group, had been sent to Mali after being selected for a counter terrorism intelligence operation.

The Naval Criminal Investigative Service (NCIS) are still piecing together the chain of events that led to his death.

According to US officials, he stopped breathing at around 5am and attempts to reopen his airway failed. He was rushed to a French clinic in Mali by another Green Beret and the two SEALs, but was already dead on arrival.

The two SEALs later told investigators Melgar was drunk and had been taking part in "combatives" – a military term for hand-to-hand fighting – in the moments leading up to his death.

But an autopsy reportedly found no drugs or alcohol in his system. His cause of death was given as "a homicide by asphyxiation", the New York Times reported.

The incident has threatened to stain the reputation of SEAL Team Six, the elite unit that carried out the 2011 raid that killed Osama bin Laden.

The two accused Navy SEALs, who have not been identified, were flown out of Mali shortly after the death and placed on administrative leave.

Melgar, a graduate of Texas Tech University, joined the Army in 2012 and was assigned to the 3rd Special Forces Group, based at Fort Bragg, North Carolina.

Representatives of both US Africa Command (AFRICOM) and US Special Operations Command (SOCOM) declined requests from The Daily Beast for comment due to the active investigation. The NCIS would not comment beyond confirming the investigation is underway.