MEXICO CITY — Since November 2014, the Mexican authorities, eager to close a dark chapter in the nation’s history, have insisted that 43 students from Ayotzinapa who disappeared two months earlier in the city of Iguala were killed by a drug gang that incinerated their bodies in a garbage dump and disposed of the ashes in a river.

But on Sunday, in the latest blow to the integrity of the government’s case, an international panel of experts who began examining the disappearances a year ago asserted that five suspects whose testimony underpinned the government’s conclusions gave confessions “under torture or cruel, inhumane or degrading treatment.” Forced confessions are not admissible in Mexican courts.

The findings not only undermined the government’s case but also further eroded the credibility of the nation’s criminal justice system. The system has been widely criticized for its handling of a matter that has come to represent the failures and corruption of the Mexican state.

“The Ayotzinapa case has put the country at a crossroads, from which it has yet to emerge, and for that it needs a strengthening of the rule of law and of the defense, the guarantee and respect for human rights,” Alejandro Valencia, a Colombian lawyer and a member of the five-person panel, said at a news conference here on Sunday.