Police are considering issuing arrest warrants for more protesters in the wake of a clash with police in the southern province of Songkhla on Monday.
A source at the Provincial Region 9 Police said investigators will check security camera footage of the clash.
People seen to have broken the law will face arrest warrants.
Police insisted they will take legal action against the protesters based on the evidence gathered.
They said officials responsible for handling the case had no intention to bully residents but they are certain more people were involved.
The source also said that at least 20 more protesters were involved in the fracas and investigators expect to spend about a month gathering evidence to seek court approval for their warrants.
The Songkhla Provincial Court on Wednesday granted bail to 15 anti-coal protesters detained in the wake of Monday's clash with police in Songkhla.
A total of 16 protesters were arrested on Monday after clashing with police officers in the province.
A 16-year-old boy was earlier released on bail.
About 500 people walked from Thepha district to submit a letter of protest to Prime Minister Prayut Chan-o-cha, who was visiting the province for a mobile cabinet meeting in Muang district, but they were stopped by police, which led to the clash.