A suspect was being questioned by police Friday for allegedly attacking an American tourist who intervened in a purse theft on a Koh Phi Phi beach.

An unidentified 17-year-old man was held as one of three suspects in the assault last week of Stephen Trimble, a 26-year-old American tourist who shared details of what happened with his large social media audience.

According to Col. Sakchai Limcharoen, commander of Krabi Police, another suspect contacted the police to say he would turn himself in today as they continue to look for the third attacker.

Trimble wrote Tuesday on Instagram that he had stopped a local from stealing a woman’s purse as she swam in the sea. “A perfect payday for a piece of shit low life thief preying on a tourist,” Trimble wrote.

After seeing the man picked up the purse and walk away, Trimble said he yelled at him, causing him to flee. Minutes later, he wrote, the would-be thief returned with friends armed with large sticks and a kayak paddle to hand down a vicious beating.

“They came straight towards me and without hesitation began swinging on me. After about 15 strikes they take off down a dark alley at the at end of the beach,” Trimble wrote in his post. He sustained injuries to his head, arms and leg.

Trimble continued, warning tourists to not leave personal belongings unattended.

“DO NOT leave your shit on the beach without watching it. Some of the locals are scumbags and the police are most likely corrupt getting a pay cut from the stolen items. Also be careful at night, especially in these places known for partying, they’re also known for higher crime rates. Be smart, stay in groups when you can, keep your valuables on you.”

He claimed the “SUPER helpful” police officers he then found told him to go to the station at 10am the next morning.

Trimble is a photographer and blogger from Florida with over 40,000 Instagram followers.