(CNN) -- Six Kenyan political leaders -- three each from opposite sides of ethnic violence that followed the country's disputed 2007 election -- were issued summonses Tuesday to appear before the International Criminal Court as part of an investigation into alleged crimes against humanity.
Top ICC prosecutor Luis Moreno Ocampo in December named the six -- Deputy Prime Minister Uhuru Kenyatta, Cabinet Secretary Francis Muthaura, former police Chief Hussein Ali, former Agriculture Minister William Ruto, ethnic radio journalist Joshua Arap Sang and opposition leader Henry Kosgey -- as suspects in organizing violence that left more than 1,000 people dead and displaced hundreds of thousands.
The disputed presidential election sparked chaos that escalated into ethnic violence pitting supporters of incumbent President Mwai Kibaki against those of challenger Raila Odinga, who was later named prime minister in a power-sharing agreement.
The court documents issued Tuesday shed more light on the prosecutor's case against the men, who are accused of crimes against humanity.
Ocampo, according to the documents, alleges that Muthaura, Kenyatta and Ali as co-perpetrators "committed or contributed to" the killings of supporters of the opposition Orange Democratic Movement, the deportation or forcible transfer of ODM supporters, the rape and other forms of sexual violence against ODM supporters, the persecution of civilians based on their political affiliation and other inhumane acts.
"The Prosecutor submitted that the crimes allegedly committed in Nakuru and Naivasha (cities) occurred in the context of a widespread and systematic attack against the civilian population carried out by the Mungiki and pro-Party of National Unity Youth against perceived Orange Democratic Movement supporters," the court documents say.
Based on evidence presented by the prosecutor, a pre-trial chamber of the ICC found reasonable grounds to support Ocampo's allegations, and said evidence presented shows that prior to the attacks in Nakuru and Naivasha in January 2008, planning meetings were held.
"The majority of the attackers had been ferried from elsewhere prior to the attack; in the period immediately preceding events, large quantities of crude weapons were brought and distributed to the attackers; and leaflets announcing the attack were circulated among the targeted population," the chamber said in the documents.
On the opposition side, Ruto, Sang and Kosgey are accused as co-perpetrators of murder, deportation or forcible transfer, torture, and persecution committed in Turbo town, the greater Eldoret area, Kapsabet town and Nandi Hills town. The crimes, according to Ocampo, "were committed by large and organized gangs of Kalenjin youth against members of the civilian population, as part of a widespread and systematic attack" due to the local population's affiliation with the Party of National Unity.
The aim, according to Ocampo, was to expel PNU supporters from the Rift Valley "with the ultimate goal of creating a uniform Orange Democratic Movement voting block."
Ocampo alleges that Ruto, Kosgey and Sang established a network of ODM representatives, members of the media, former Kenyan police and army forces and local leaders to help carry out the plan.
"There are reasonable grounds to believe that the network was under responsible command and had an established hierarchy, with Ruto as leader, Kosgey as deputy leader and treasurer and Sang as responsible for communicative purposes," the pre-trial chamber found.
As a result of the prosecutor's evidence, the chamber summoned all six men to appear before the court in The Hague, Netherlands, on April 7.
Muthaura said in December that he would voluntarily go to The Hague if summoned by the pre-trial judges.
"The suggestion that I've done anything to warrant criminal investigation, it is manifest nonsense," said Muthaura. "This has slandered my reputation and is unjustified."
Kenyatta, a 2002 presidential candidate and son of the nation's first president, also serves as the finance minister.
"I don't think it will have much impact, our work will continue, the government will continue, we are a mature country that can handle its issues," Kenyatta said in December after he was named on the list of suspects.
The Kenyan government has been working to defer the ICC case by promising to set up a special tribunal to investigate the alleged abuses. So far, the government has not formed a tribunal.
"We don't want to let foreigners drag this country to the dogs," Beth Mugo, the Kenyan health minister, said in December. "We have come to a crossroads, do we want to keep our country sovereign or do we want our country to become a failed state?"
CNN's David McKenzie contributed to this report.