* This story has been updated.
An estimated 40 people were killed in a gun battle today after armed civilians ambushed a federal police convoy in western Mexico, a government official told VICE News.
The shootout occurred in a community called Tinaja de Vargas, in the municipality of Tanhuato, Michoacan state, near the border with Jalisco. Gunfire started at around 10 am or 11 am, reports said.
Gonzalo Ponce, a spokesman for the federal interior ministry, initially said it was still unclear who the attackers were. The western region of Jalisco and Michoacan and its drug trade have been heavily disputed in recent years by the New Generation Cartel of Jalisco and the rival Knights Templar.
Michoacan Governor Salvador Jara later said several arrests were made and the Jalisco New Generation Cartel was "mostly likely" involved in the shootout.
The gunfire reportedly began when a federal police convoy came under fire while investigating reports that armed men had been spotted in the area. Another gun battle took place at Rancho El Sol, a nearby property on the outskirts of Tanhuato.
Thirty-nine people were killed in the confrontations, including one federal agent, Ponce said, although the number was not fully confirmed.
Other reports said 43 or 44 people were killed.
The high number of casualties would make it one of the worst single incidents of gun violence in recent memory in Mexico, which declared war against drug cartels in the country in late 2006.
Police reportedly recovered over 40 firearms, including grenade launchers and AK-47 and AR-15 assault rifles, as well as ammunition and radio equipment apparently used by the aggressors.
Mexico's national security cabinet was headed on Friday afternoon to Tanhuato to investigate the incident.
The one-sided nature of the initial death toll and photos that appear to show several unarmed victims raised speculation on social media that Friday's incident was not a conventional shootout.
Just four days ago, the Michoacán state government and Mexico's army sent reinforcements to boost security in the area, following the murder of a mayoral candidate in the neighboring town of Yurécuaro on May 14.
A former member of the local vigilante or autodefensa group, Enrique Hernandez Salcedo, running for the leftist Morena party in next month's elections, was shot to death by four armed men aboard a moving vehicle while at a campaign rally.
The previous mayor of Tanhuato, Gustavo Garibay Garcia, was also shot dead outside his home by three assailants in March 2014.
Founded in 2010, the CJNG, as the cartel is known, has grown increasingly confrontational in recent weeks, killing dozens of police officers in a series of ambushes and confrontations across Jalisco.
Tanhuato lies just across the border from the town of La Barca, where the bodies of 67 suspected victims of the cartel were found in mass graves in late 2013.
Reporters Duncan Tucker in Jalisco and Juan Jose Estrada Serafin in Michoacan contributed to this report. Check back for updates on this story @VICENews.