Mexico: The arrest of an ex-mayor and wife team suspected of masterminding the disappearance of 43 students raised hopes Wednesday that authorities can finally track them down -- dead or even alive.

Thousands of security forces backed by drones and boats have been combing towns, mountains and rivers in the southern state of Guerrero in a frantic search for the missing young men.

But they have yet to find any trace of them since they vanished almost six weeks ago in the city of Iguala after an attack by local police linked to the Guerreros Unidos drug gang.

Instead, authorities have discovered a dozen mass graves containing 38 bodies, raising fears of a tragic end to a mystery that has blown into a crisis for President Enrique Pena Nieto.

Officials say preliminary DNA analyses have shown that at least 28 of them are not the students from the teacher-training college of Ayotzinapa, but that they were waiting for final results from independent Argentine forensic experts.

Despite the gruesome discoveries, parents of the missing and some officials harbor slim hopes, with Guerrero`s interim governor saying the abductors are possibly moving them around to elude the search.

Officials hope the capture of Iguala`s ex-mayor, Jose Luis Abarca, and his wife Maria de los Angeles Pineda will lead to the students.

After more than a month on the run, Abarca and Pineda were captured in an abandoned house of a gritty Mexico City district on Tuesday.

The pair are accused of ordering Iguala`s police to attack busloads of students over fears they would disrupt a speech by Pineda on September 26, sparking a night of violence that left six people dead and the 43 young men missing."The capture of Abarca makes us believe that the biggest piece of the puzzle can shed more light on some of the scenarios that we have been building," Guerrero interim Governor Rogelio Ortega told the Televisa network.

Ortega said witnesses have told graduates of the students` teacher-training college that the 43 were taken near the town of Teloloapan, west of Iguala, and then Cuetzala before being split into two or more groups.

"These are clues, comments that feed hope and faith," said Ortega, whose predecessor Angel Aguirre resigned last month amid violent protests over his handling of the case.

"They are hiding them, moving, evading the search," he said while stressing there is "uncertainty" over whether they are alive.

Alejandro Hope, a security expert and former intelligence officer, said the most likely scenario is that the students were killed.

"Why would they hold 43 people for 40 days without asking a ransom? It makes no sense," he told AFP.

Saying the students may be alive is an attempt by Ortega to "manage the conflict" and "wait for the social protests to calm down," Hope said.

Recent high-profile abductions have ended tragically.

Last year, 13 young people who were snatched from a Mexico City bar were found dead in a mass grave three months later.

Last week, authorities in the northern state of Tamaulipas found the bodies of three American siblings and a Mexican man two weeks after they disappeared.Resolving the Iguala mystery has become a matter of urgency for Pena Nieto, who has met parents furious at the lack of results and faced angry protests, including a new one planned for Wednesday.

Pena Nieto said he hoped the arrest of Abarca and Pineda would contribute to the investigation "in a decisive manner."

Authorities have detained 59 people in the case, including Guerreros Unidos members and 36 police officers from Iguala and the neighboring town of Cocula.

Suspects have told investigators that 17 students were killed, but their testimony has not been confirmed.

Prosecutors say Iguala`s police handed the students to Cocula colleagues, who in turn delivered them to the Guerreros Unidos.

After that the trail went cold.