(CNN) -- One of four former Khmer Rouge leaders charged with crimes against humanity was ruled unfit to stand trial and could be set free, a spokesman for the special U.N. court in Cambodia said Thursday. 

Lars Olsen said the Extraordinary Chambers in the Courts of Cambodia decided that Ieng Thirith, the 78-year-old former minister of social affairs in the Khmer Rouge regime, was "not fit to stand trial as she has dementia." 

Prosecutors have 24 hours to object to the court's decision, Olsen said. If there is no objection, Ieng Thirith will be released, he added. 

Opening statements by the defendants are scheduled for Monday. 

Ieng Thirith was the only woman among the four defendants in the trial, which charges surviving leaders of the four-year Khmer Rouge regime in the 1970s with crimes against humanity, grave breaches of the Geneva Conventions, genocide, homicide, torture and religious persecution. 

The other defendants are her husband, Ieng Sary, the former Khmer Rouge foreign minister; Khieu Samphon, the nominal head of state; and Nuon Chea, the prime minister, also known as Brother Number 2. 

The head of the Khmer Rouge, Pol Pot, was known as Brother Number 1. He died in 1998, long before the U.N.-backed court came into existence. 

Under Pol Pot's leadership, the Khmer Rouge regime was responsible for the deaths of millions of ordinary Cambodians during a four-year reign of terror that was eventually halted in 1979 by invading Vietnamese forces. 

In 1975, the Khmer Rouge ordered people out of Phnom Penh, the capital, and other cities in Cambodia to work in the countryside. 
When did it end?
four-year reign of terror that was eventually halted in 1979