The reforms would allow the legal cultivation of marijuana on Mexican soil after decades of drug war violence.
Mexican officials have arrested a dozen police officers for suspected involvement in the killing of 19 people in the northeastern state of Tamaulipas last month.
Irving Barrios, attorney general of Tamaulipas, told a news conference on Tuesday investigations showed at least 12 state police were likely involved in the killings in the municipality of Camargo.
Authorities have so far identified two Guatemalan migrants among the 19, whose bodies were found badly charred in a bullet-riddled, burned-out vehicle.
The detained police officers appeared to have altered the scene of the crime, removing ammunition casings, prosecutors said. A police report and information the suspects gave in interviews did not match phone records and location data, they added.
Of the 19 bodies examined, 16 were males, one was female, and the two others were so badly burned their gender had not yet been determined.
The killings caused renewed consternation in Mexico about perils faced by migrants, many of whom suffer abuses on the journey north from impoverished Central American countries.
If the involvement of law enforcement officials is proven, the case would echo the abduction and apparent massacre of 43 student teachers in the southwestern city of Iguala in 2014 by corrupt police working with a drug gang.
President Andres Manuel Lopez Obrador has promised to put an end to killings and kidnappings by authorities, but ties between security forces and organised crime have been hard to stamp out.
The two Guatemalan migrants were identified after their Maya Indigenous relatives gave DNA samples to help investigators.
More than a dozen victims are believed to be from Guatemala’s highlands, a region hard hit by the effect of the coronavirus pandemic and extreme weather last year.
Also among the dead was the Mexican owner of one of the burned-out pick-up trucks found at the scene, prosecutors said. The truck had been impounded weeks before during the detention of 66 migrants in the neighbouring state of Nuevo Leon, but was later released to the owner, identified only as “Jesus M”.
Daniel P, another of the Mexican victims, was a known migrant smuggler, prosecutors said.
The killing is the latest chapter in Tamaulipas’s history of police corruption. Most towns and cities in the state saw their municipal police forces dissolved years ago because officers were often in the pay of the cartels.
A more professional state police force was supposed to be the answer, a belief that came crashing down with the arrests announced on Tuesday.
The attack also took place in a rural area along the busiest stretch of the US-Mexico border for illegal crossings.
Every year, tens of thousands of migrants, many from Central America, move through the region under the watch of guides who pay cartels for permission to transit. The lucrative routes are disputed by splinters of the Gulf Cartel and feared Zetas gang.
Investigators say the victims were part of a larger convoy that included other vehicles carrying migrants, including Salvadorans, and another with armed men on board apparently for protection.