IGUALA INDEPENDENCIA: Buses rumble out of Iguala's central station in southern Mexico packed with passengers and, sometimes, drugs hidden inside egg cartons, which may have been the motive behind last year's disappearance of 43 students.

Every day, some 60 buses bustle in and out of the Estrella Blanca station next to a crowded market, in a city that became known as a bastion of the Guerreros Unidos drug cartel following the tragedy.

But it was only this week that the use of passenger buses by the gang emerged as a potential motive, following a six-month investigation by independent experts from the Inter-American Commission on Human Rights.

On the night of September 26-27, the students from the Ayotzinapa teacher college hijacked five buses before they were attacked by municipal police on the mayor's orders, according to prosecutors.

Taxis wait outside the bus station of Iguala de la Independencia, Guerrero State, Mexico, on September 9, 2015. — AFP

Authorities said the officers, who were linked to Guerreros Unidos, handed the students over to the gang, which killed the young men after confusing them with rivals.

But the independent investigators issued a report last Sunday urging prosecutors to probe the possibility that the students — radicals who usually hijack buses to travel to protests — were abducted because they inadvertently seized a bus laden with drugs.

The commission's report looked into that theory after they discovered the existence of a fifth bus that was mysteriously never mentioned in the official investigation, which had only referred to four.

Government officials said prosecutors did explore the possibility of a drug bus being the motive, but that it was never made public because it was a "very delicate" issue.

To back their theory, the independent experts cited a US federal case against eight suspected Guerreros Unidos members in Chicago, who were formally accused in December of using buses to ship drugs from Mexico.

The US criminal complaint does not say where the drugs originated in Mexico, but it shows that suspects phoned from an area code used in the Iguala region.

'Take this package'

An official in the Mexican attorney general's office told AFP that the Guerreros Unidos hide opium paste inside boxes of eggs or detergent that drug "mules" carry with them on buses.

"The packages are very easy to hide," the official said on condition of anonymity, adding that once the drugs reach Chicago, the cartel uses buses again — this time to bring the cash from the proceeds to Mexico.

Two drivers interviewed by AFP at the Iguala station confirmed that gangs have used buses to ship narcotics out of the city, though both said they never worked with them.

They tell you "Take this package and deliver it," said a driver for the Estrella Roja bus line, which owns the mysterious fifth bus mentioned by the independent investigation.

Estrella Roja buses travel to the central state of Morelos, a region where members of the Guerreros Unidos have been detained in connection with the Iguala case.

The other veteran bus driver, who works for the Costa Line company, said the drugs are sometimes hidden in the driver's cabin.

"It's not that easy any more because the federal police conduct unannounced inspections with dogs," said the driver, whose company owns two of the other buses that were used by the Ayotzinapa students.

Costa Line and Estrella Roja company representatives declined to comment.

Did cops know?

"It's a rather simple method to hide drugs, which requires very little volume but which is very lucrative," said Alejandro Hope, a former Mexican intelligence services official.

"Bus stations have much less surveillance than airports," said Hope, now the security editor at ElDailyPost.com.

Federal police sporadically visit the Estrella Blanca station while five employees from a private security firm check luggage without the help of X-ray machines or dogs.

A man who identified himself as a member of the Guerreros Unidos told AFP that the cartel has had as many as three shipments per week out of Iguala, carrying as much as 90 kilograms of narcotics.

Soldiers patrol a street in front of a mural depicting the designer of the first Mexican flag Agustin de Iturbide (L) and Mexican revolutionary leader Vicente Guerrero (R) in Iguala de la Independencia, Guerrero State, Mexico on September 9, 2015. — AFP

The drugs are stuffed in boxes of 360 eggs that are taken to various parts of the country, said the scar-faced man, whose membership in the gang could not be independently verified by AFP.

Iguala municipal officers knew that buses were used to transport drugs, he said, adding that his cohorts told him that the night that the students vanished, "several buses were carrying drugs."

The independent investigation cast doubt over the fate of the students, saying that contrary to what prosecutors said, there was no evidence that they were killed and incinerated in a garbage dump.