Somchai Neelapaijit, who went missing on 12 March, had offered legal aid to five Muslim men accused of involvement in a raid on a Thai army arsenal on 4 January in which four soldiers were killed and hundreds of weapons were stolen.
Somchai's disappearance and a widespread belief that he was murdered by police have been cited as among the causes for an upsurge in anti-government violence in Thailand's predominantly Muslim southern provinces.
More than 180 people have been killed this year in clashes between government forces and Islamic separatists and in targeted killings by Muslim rebels.
Court officials said the five policemen arrested in connection with Somchai's disappearance, including a major, two lieutenant-colonels and two sergeants, on Monday denied involvement in the alleged kidnapping.
They were ordered to appear in court again on 12 July when the prosecution was to present its case, followed by a trial if the evidence is deemed sufficient by the court.
At a ceremony on Sunday to mark the 100th day since Somchai's disappearance, senators and human rights advocates questioned the government's sincerity in arresting the five policemen.
"Thailand has become a police state"
director, law centre, Thammasat University
"Thailand has become a police state," the Bangkok Post quoted Banjerd Singkaneti, director of Thammasat University's Law Centre, as saying. "We do anything to achieve a goal, without caring about the methods."
Senator Sak Koesaengrueng called on Prime Minister Thaksin Shinawatra to tell all what he knows about the case.
Earlier Thaksin was quoted as saying the missing lawyer was believed to have been kidnapped and taken to the northern province of Mae Hong Son.
But Thaksin has declined an invitation to provide details to a senate committee investigating Somchai's disappearance.