Mexican President Enrique Pena Nieto met with parents of 43 teachers college students Wednesday for the first time since they disappeared over a month ago, when investigators say police detained the students and handed them over to a drug gang.

The mothers and fathers of the missing have grown increasingly frustrated at the pace of the investigation of the Sept. 26 police attack in the city of Iguala, which also left six dead. The case has shaken the image of improving security that the government has sought to project since Pena Nieto took office in 2012.

The parents were inside the Los Pinos presidential residence for more than six hours, and at one point human rights officials said the meeting was over but the family members were refusing to leave until Pena Nieto signed a document that would satisfy everyone.

Pena Nieto said later in televised remarks that he heard their concerns and agreed to a number of points they raised. Those included setting up a commission made up of both government officials and parents to provide daily updates on the investigation, providing greater support for both rural teachers colleges and the families of those who were killed or wounded in the attack, and redoubling efforts to determine the students' whereabouts.

"There will be a renewed search plan," Pena Nieto said.

Even before the president's remarks, one of the parents, Felipe de la Cruz, said on the others' behalf that the president's response was "not sufficient."

De la Cruz spoke by phone to reporters gathered at a human rights center, where parents later appeared and talked with journalists.

The relatives came to Mexico City from the Rural Normal School of Ayotzinapa in Guerrero state, where the missing students were enrolled.

Investigators have said the attack was ordered by the former mayor of Iguala over fears that the students planned to disrupt a speech by his wife. Police officers are alleged to have turned the students over to the Guerreros Unidos drug cartel.

Forensic experts have combed through 11 clandestine graves discovered in the area during the hunt for the students. The sites contained 38 bodies, but there has been no confirmation that any of them belong to the missing students.

Rafael Lopez, father of one of the missing, expressed frustration at authorities' inability to determine what happened.

"If they can't do it, they should ask for help from the United States," Lopez said from the school, where he and some of the parents remained instead of traveling to the capital.

Iguala's former mayor and his wife are fugitives, and 56 people have been detained, including police from Iguala and the nearby town of Cocula as well as the alleged leader of Guerreros Unidos.

Pena Nieto said the rule of law will apply regardless of "wherever (the investigation) leads."

The case has spurred anger across the country, and large protest marches demanding action by the government have been held in Mexico City, Acapulco and the capital of Guerrero, Chilpancingo.

Several protests have erupted in violence. On Wednesday, protesters aligned with the teachers college attended by the missing attacked the Guerrero governor's residence Chilpancingo, damaging the front gate and burning an official vehicle outside.