Heckler & Koch, Germany's biggest manufacturer of small arms, may have supplied some of the weapons used in the September killing of several students in Iguala, Mexico. According to a report published this week by German daily "taz," Mexican investigators seized 36 rifles made by the German firm from local police - weapons that should never have been exported into the region in the first place.

Six student-teachers were killed in Iguala in the state of Guerrero on September 26 when police fired at buses carrying more than a hundred people on their way to a demonstration. Later, a number of students were allegedly rounded up by police and handed over to the criminal gang Guerreros Unidos. Of those, 43 are still missing and are thought to have been murdered - a theory supported by the DNA identification of the remains of one of the missing students found in a mass grave.

Taz reported it had a list of weapons impounded by investigators, which if verified would prove that Heckler & Koch's rifles were illegally sold to Mexican police forces. Mexico has seen mass protests since the presumed massacre of the students, and President Enrique Pena Nieto has come under pressure intense enough to affect a regional summit with other Latin American leaders.

Mexico has seen mass protests since the disappearance of the students

Investigators have arrested 80 people in connection with the killings, and have seized 228 firearms from police, of which, according to taz, 36 were H&K's G36 assault rifles. The G36 is the German military's standard weapon and considered one of the deadliest firearms in the world. Investigators have said 30 weapons were used at Iguala, though it remains unclear which.

Selling to corrupt regimes

The German government approved the sale of around 9,500 G36 rifles to Mexico in 2007, on condition that they would not be used in four states, including Guerrero, where security forces were considered particularly corrupt. In 2010, media reports showed the rifles were being used by security forces there, and alleged that H&K employees had helped trained officers to use them. Nearly 2,000 G36 rifles are in the inventories of the Guerrero police.

In 2010, activist Jürgen Grässlin filed criminal charges against the company in Stuttgart for violating export laws, but Stuttgart prosecutors have failed to bring the investigation to court.

"Experience tells us that Heckler & Koch's assault rifles can be fully functional for around 50 years," Grässlin told DW. "The consequences for Mexico will be fatal. In these first years, extremely corrupt Mexican police officers will shoot and kill demonstrators and other defenseless people with these illegally exported German assault rifles. In the years and decades to come the drug mafia there will use them after buying or taking them from the police."

Grässlin's speculation is backed up by other incidents. In December 2011, two demonstrators were shot dead by police in Chilpanchingo, the capital of Guerrero. While the murder weapons could not be established, investigations at the time found that officers were carrying G36 rifles during the operation. According to Mexico's chief prosecutor, Jesus Murillo, police in Guerrero had been receiving the equivalent of 50,000 euros ($62,000) per month from the Guerreros Unidos.

Delayed investigation

In early December, H&K, based in the tiny town of Oberndorf in the southern German state of Baden-Württemberg, settled a legal row with two employees it accused of being responsible for the Mexico deal - after the pair had successfully sued the company for wrongful dismissal earlier this year. That court case included revelations that employees had forged documents to disguise the sale. But, despite the fact that Stuttgart state prosecutors have been investigating the company for four years, criminal charges have not been filed.

Heckler & Koch is one of the world's biggest small arms manufacturers

"I am perplexed as to what could have taken four years for this investigation," said Jan van Aken, a Left party Bundestag member who has long lobbied the German government to clamp down on weapons exports. "The state prosecutors urgently need to press charges, and put all the evidence on the table, and then maybe we can put the investigation from Mexico in the mix too. Then maybe we might be able to investigate whether some of these murders were carried out with Heckler & Koch weapons. But four years is intolerable."

H&K did not respond to a request for comment on the matter. Grässlin said he expects state prosecutors to press charges in spring 2015, following the release of a report from the German customs investigation bureau. "There are at least 14 people involved, including some from management level," he said.

"In my opinion, the facts that would be enough to press charges have been known for four years," van Aken told DW.