A police statement said Mihigo, 38, committed suicide.
Mihigo was sentenced to 10 years in prison for offences including the formation of a criminal gang, conspiracy to murder, and conspiracy against the established government or the president, according to Human Rights Watch.
In 2018, he was granted a presidential pardon by Kagame.
Last week, however, he was rearrested when crossing the border into Burundi, with police saying it was a breach of the conditions of his pardon.
“He had been in a police cell for three days as police investigated why he was crossing the border illegally and cases of bribery,” Police spokesperson Eric Kabera said in the statement.
According to the police, Mihigo had been allowed to meet family members and his lawyer.
It is not known whether he was being held in solitary confinement.
A devout Catholic known for his songs of forgiveness, Mihigo is a cultural icon in Rwanda and his death has been met with disbelief.
“Mihigo was somebody who is down to earth and loved by many. News about his death is a shock to the country,” fan William Murinzi said.
