"The Inter-American Commission deeply regrets the Mexican government's decision of not supporting, of not extending our mandate in spite of the fact that our objectives remain unfinished," said James Cavallaro, the commission's president.
The human rights group commissioned the panel to help find out what happened to the students from a rural teachers' college who went missing in the Mexican city of Iguala while on a trip to raise money for activities.
Their fates remain mostly unknown.
Shortly after the students disappeared, then-Mexican Attorney General Jesus Murillo Karam said the young men were abducted on orders of a local mayor and turned over to a gang that killed them, burned their bodies to ashes in a landfill and tossed the remains into a nearby river.
The Mexican government had already put an end to the panel's investigation by not extending the six-month agreement made with the commission and the parents of the missing in September.
Panel members accused the Mexican government of undermining the probe by, among other things, not letting them re-interview suspects, not giving them access to key information and ignoring investigative angles the experts had suggested.
Panel member Claudia Paz said there was a "probable link" between what happened to the students and a heroin drug route connecting the city in Guerrero state to Chicago, a line of investigation that has not been fully explored.
"Unfortunately, as was already mentioned, we could make very little progress along this line of investigation nor could advances be made in the investigation of state workers we had requested provide their statements," Paz said.
Both the panel and a group of independent forensic experts from Argentina
have cast doubts on the official conclusion that the students were kidnapped, killed and their bodies burned.
The groups concluded in separate investigations that there was not a fire that morning of the magnitude necessary to incinerate 43 bodies.
Forensic experts have been able to identify the remains of only one student, Alexander Mora Venancio
, who was 19 at the time of his death. He was identified using a bone fragment found inside a trash bag in a river near the landfill where Mexican officials say the other students' bodies were incinerated.
Mexican Deputy Attorney General Eber Omar Betanzos rejected the group's arguments that the government has blocked the investigation.
"The Iguala case represents the most comprehensive criminal investigation in the history of law enforcement in Mexico. It has also had unprecedented international collaboration, transparency and work with the victims," Betanzos said.
He said "the families of the students and the government are on the same side and we worked for the same objective -- to know what happened to the students and punish each person who was responsible."
Mexican President Enrique Peña Nieto said on Twitter that the Mexican attorney general's office "will analyze the full report to enrich its investigation into the tragic event of 26 and 27 September 2014."
"The AGO will continue to work for justice with openness, responsibility and adherence to the rule of law," Peña Nieto said in a second tweet
The parents have organized to put pressure on the government. They have held protests across Mexico, mainly in Mexico City and Guerrero, the states from which most of the students and their families originate. Some of the demonstrations have turned violent.
Emiliano Navarrete, the father of one of the students, told CNN in September he still hopes to bring his son back alive.
Jose Angel Navarrete was 19 when he went missing.
"Believe me! I will bring him back. He will come back one day," Navarrete said on the first anniversary of his son's disappearance.