Forensic experts in the city of Surat Thani have said that the explosives used in the wave of bombings that struck Thai resort towns last Friday are similar to those used by separatists in the country’s Deep South.

While stressing that it was too early to say with certainty who was behind the violence, Police Lieut. General Suchart Theerasawat told the Bangkok Post, “The bombs used in the Phuket, Phang Nga and Surat Thani attacks were related and similar to those found in insurgent attacks in the Deep South.”

Nobody has so far claimed responsibility for the bombs, which killed four people and injured dozens in Surat Thani, Hua Hin, Phuket and other locations with above-average concentrations of foreigners. Incendiary devices and arson are also suspected of being behind the simultaneous fires that gutted market and shopping facilities popular with tourists.

The Post quoted unnamed military intelligence sources as saying they were “confident” that the attacks were a “show of force and network expansion” on the part of the Barisan Revolusi Nasional, a Muslim separatist group.

However, a senior junta member, retired army commander in chief Prawit Wongsuwan, told reporters Monday that the attacks were not an expansion of the southern insurgency. According to Reuters, he said that insurgents could have been hired by other parties to commit the violence.

Since 2004, at least 5,000 people have died violently in the insurgency, which is mainly being fought in Thailand’s three southernmost provinces. On Sunday, two soldiers were injured, one of them seriously, when a roadside device exploded in Narathiwat province.

Last Friday’s bloodshed coincided with the birthday of Thailand’s Queen Sirikit (the holiday also marks Mother’s Day in Thailand). It also occurred near the first anniversary of the bomb attack on Bangkok’s Erawan Shrine, a major tourist attraction. That attack, on Aug. 17, 2015, killed 17 and injured over 100.

Speaking to TIME about the bombings last week, Zachary Abuza, an expert in Southeast Asian politics and security at the National War College in Washington, D.C., said that by targeting tourist areas, the bombers were “trying to do harm to the Thai economy. That’s the Achilles heel of the junta.”

Thai police have arrested two people in connection with attacks. Reuters reports that “several others” have been detained for questioning, but details have not been released.

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