Mexican authorities on Friday said a municipal police chief was arrested in connection to the killing of nine family members of a Mormon breakaway group in northern Mexico last month.

The mayor of Janos told local media that Fidel Alejandro Villegas, municipal police chief of Janos, was taken into federal custody for suspected links to the crime and consequently transferred to Mexico City.

Although not officially confirmed by federal authorities, his arrest is considered the third of its kind over the massacre.

Mexican authorities believe the gang that killed the family had mistaken their SUVs for those of a rival gang.

The victims were all US-Mexican dual nationals and formed part of a breakaway Mormon group that settled in the area decades ago.

Read more: Mexico: When drug violence 'turns into terrorism'

Three women and six children were killed by unknown assailants

Under pressure

The United States placed significant pressure on Mexico to include the FBI in the investigation, with US President Donald Trump saying his government "stands ready, willing and able to get involved and do the job quickly and effectively."

"The cartels have become so large and powerful that you sometimes need an army to defeat an army," Trump said in a tweet at the time.

Despite pressure from the victims' relatives, the US did not label drug cartels operating in Mexico as terrorist organizations.

Read more: El Chapo: True drug bosses are still at large

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ls/sms (Reuters, EFE)