N'Djamena - Chadian officials say they have opened an investigation into the disappearance of more than 20 soldiers in the days before and after the April 10 presidential election.Prosecutors announced the probe late on Monday, after calls for action from right groups and, more recently, France.Investigators have already interviewed families of those reported missing, hospital staff, rights groups and others, state prosecutor Alghassim Khamis said.The soldiers went missing as President Idriss Deby was re-elected for a fifth term after 26 years of rule. Troops voted a day ahead of the poll.On April 28, the Chadian League for Human Rights (LTDH) and Amnesty International said more than 20 soldiers and security personnel were feared missing and called for government action. They were thought to have resisted pressure to vote for Deby, they said.Some news reports in Chad have put the number of missing members of the defence and security forces as high as 60. The Tchadconvergence website said the bodies of four dead soldiers had already been found.Last week, the French foreign ministry expressed its concern about the disappearances and called on the Chadian authorities to clear up the matter.Deby's election victory last month was marred by a clampdown on demonstrations by unions and rights groups calling for a change of leadership and democratic reforms. Several leading activists were arrested and some have since left the country.