MEXICO CITY (Reuters) - Unidentified gunmen shot dead a trade union leader in Mexico’s central city of Salamanca, authorities said on Sunday, the latest outbreak of violence in an area plagued by a murderous struggle between gangs over fuel theft.

Gilberto Munoz, a former federal lawmaker and the leader of the national chemical and petrochemical union, died after being shot on Saturday, the government of Salamanca’s home state of Guanajuato said in a statement.

Salamanca is home to one of Mexico’s six oil refineries, and the facility has been under the protection of the army since the start of the year after President Andres Manuel Lopez Obrador launched a crackdown on fuel theft across the country.

The motive for the attack on Munoz was unclear. A spokeswoman for Guanajuato’s government said investigations were ongoing and that no further details could be revealed for now.

The fight for control of access to pipelines has turned the once relatively peaceful state of Guanajuato, a center of the carmaking industry, into one of the most violent.

Between 2007 and last year, the annual murder tally in Guanajuato leapt more than ten-fold to over 2,600.

Taking office in December, Lopez Obrador pledged to reduce the violence in Mexico. So far, murders are still rising.

A total of 969 murder investigations were opened in Guanajuato in the first four months of his administration, a jump of 50 percent from the same period a year earlier, according to figures published by the interior ministry.