Hani, a hugely popular figure and the then leader of the South African Communist Party (SACP), was gunned down by Janusz Walus, a white supremacist, on April 10, 1993.
Three decades on, many South Africans harbor questions about the killing, suspecting Walus and his accomplice did not act alone.
Conspiracy theories, involving anyone from the secret services to the ANC, abound.
"I don't have closure," Hani's widow, Limpho, told a memorial ceremony attended by SACP party leaders and foreign dignitaries on Monday.
"That is why I am wearing black today. Until such time the truth comes out I am in mourning for life."
Last week, the SACP called for a fresh inquest into the murder, with a petition hoping to collect 30,000 signatures.
"There were many... factors that were not properly investigated," SACP's leader, Solly Mapaila told AFP on Wednesday. "We need to know the truth."
Aged 50, Hani was shot dead in the driveway of his home in eastern Johannesburg in front of his 15-year-old daughter.
The incident led to protests and rioting in black townships.
As anger within South Africa's black majority reached boiling point, Nelson Mandela appeared on national television to appeal for calm.
The move helped ease tensions and open the way to South Africa's first multi-racial elections the following year.
Walus's and his accomplice, Clive Derby-Lewis, had hoped to spark a racial conflict but were quickly arrested.
Derby-Lewis was released in 2015 on medical parole after 22 years in jail. He died of lung cancer in 2016, aged 80.
Walus was released on parole in December last year in a controversial decision.
"The democratic government, which my husband died for, has betrayed Chris and his family by releasing his assassin," Limpho Hani told the memorial ceremony on Monday.
"The killer is free. And the opportunity for full truth on the wide conspiracy of Chris Hani assassination is now buried and lost completely."
New Hani Murder Probe Sought
JOHANNESBURG - South Africa on Monday commemorated slain anti-apartheid activist Chris Hani amid calls for a fresh probe into the murder that almost plunged the country into a race war 30 years ago.
