MEXICO CITY (Reuters) - Gunmen shot dead 13 people at a bar in the city of Minatitlan in the Mexican Gulf coast state of Veracruz, authorities said on Friday, in one of the worst slayings to hit Mexico since President Andres Manuel Lopez Obrador took office.

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The unidentified assailants opened fire on Friday night after coming to look for a man at a bar in the southeast of Minatitlan, a spokesman for the government of Veracruz said.

Seven men, five women and a child died in the shooting, which occurred close to Minatitlan’s oil refinery, one of six run by state oil firm Petroleos Mexicanos (Pemex). Four other people were injured, the state government said in a statement.

The motive for the killings was unclear, the spokesman said.

The man the gunmen were seeking was identified as the owner of a bar in the city, the state government said. The attack took place during a family celebration.

It was not immediately clear if the man owned the bar where the attack occurred, nor whether he was present at the time.

Hugo Gutierrez, the head of public security in the state, said on Twitter that an operation had been launched to capture the people responsible for the killings.

The oil-rich state of Veracruz has been convulsed by gang violence and political corruption scandals for several years.

Lopez Obrador took office in December vowing to reduce violence in Mexico, where more than 200,000 people have been killed since the end of 2006 in brutal turf wars between drug cartels and their clashes with security forces.

After reaching record levels in 2018, murder rates have stayed high, surpassing previous-year levels in the first three months of the new government, official government data shows.

The president was due to visit Veracruz on Sunday, according to an official schedule published before the attack took place.