Authorities in northwestern China have cordoned off an area twice the size of New York City as their hunt for the perpetrators of one of the region’s deadliest ever attacks enters its fourth week.

Residents of Baicheng county in Xinjiang say that at least 60 people, most of them migrant coal miners from across the country, were knifed to death in a September 18 attack on the Sogan coal mine.

Most of Baicheng’s residents are Muslim Uighurs.

Xinjiang has long been a strategic priority for the Chinese government because of its natural resources, including the country’s largest coal reserves, and its proximity to even bigger energy sources in Central Asia. It is also a key component of President Xi Jinping’s “New Silk Road” strategy, aimed at enhancing Eurasian infrastructure links.

“More than 60 people were killed at the mine,” one Baicheng resident told the Financial Times by telephone. “Military helicopters and drones are still searching the mountains for the attackers.”

The helicopters and drones are operating out of the airport at Aksu, the largest city in the area. Police have established checkpoints on all roads leading to Baicheng, which covers an area of about 16,000 sq km. Heavily armed police are posted behind sandbag bunkers at each road block, providing cover for their colleagues who perform identification and weapons checks on all people entering the area.

A Financial Times reporter was turned back at one of the checkpoints on Saturday. “You can’t go to Baicheng or the coal mine because of the counter-terrorism operation,” one police officer said. He declined to comment further on last month’s attack, which was first reported by Radio Free Asia’s Uighur-language service, or the continuing search. According to RFA, last month’s victims included at least five police officers who responded to the incident.

Despite the information blackout, the attack is the talk of Aksu, where residents speculate that the death toll may exceed 100.

Organised attacks on symbols of the state, especially police stations, have been common for years in Xinjiang. But beginning in March last year, alleged Uighur separatists began to deliberately target civilians in well coordinated assaults, at a train station in the city of Kunming in southwestern China, and a food market in Urumqi.

Last month’s Baicheng massacre would appear to fit this pattern as most of the victims were migrant coal miners, but it is also possible that apparently sectarian-motivated attacks in Xinjiang could instead stem from disputes over pollution or land.

Nick Holdstock, who has written several books about Xinjiang, said: “A number of these [recent] incidents have differed from previous ones in that they seemed to target civilians and involved a degree of planning. But we should be careful not to assume that people in different regions in Xinjiang share the same grievances simply because of their shared ethnicity. The concerns of Uighur businessmen may not be the same as Uighur farmers.”

The Chinese government claims that shadowy terrorist groups and religious extremists are responsible for much of the violence.