The strange case of President Obama’s bogus sign-language interpreter has become even stranger — and a lot grimmer.

Thamsanqa Jantjie, who ignited outrage by signing gibberish at Nelson Mandela’ memorial service in Johannesburg, was part of a mob that burned two men to death in a “necklacing” attack, four people close to him said Monday.

But Jantjie was never tried for the 2003 slayings because South African authorities determined he was not mentally fit, they said.

Instead, Jantjie — who stood just three feet from Obama and other dignitaries during Tuesday’s service — was institutionalized for more than a year.

The bizarre new revelation comes from one of Jantjie’s cousins and three of his friends. They told The Associated Press he was part of a group of men that had found two men with a stolen TV in 2003.

The mob “necklaced” the two men, setting fire to tires placed around their necks. The practice was used by mobs to kill blacks suspected of aiding the white regime during the struggle against apartheid.

Speaking anonymously, the cousin and the three friends said the necklacing occurred within a few hundred yards from Jantjie’s home outside Soweto.

They said the others in the mob were tried in 2006 but Jantjie was ruled unfit.

Jantjie hinted at such an incident in an interview published Sunday by the Sunday Times newspaper of Johannesburg.

“It was a community thing, what you call mob justice, and I was also there,” he told the paper.

Last Thursday, Jantjie said he had been violent “a lot” but declined to elaborate and blamed it on his schizophrenia, for which he said he was institutionalized for 19 months, including in 2006.

He also blamed his meaningless hand flapping on a schizophrenic episode in which he saw “angels” descending onto the service in FNB Stadium.