Authorities say they have found no evidence linking the mayor of tambon Ban Thi municipality in Lamphun with documents containing misinformation on the draft charter.
This follows the arrest on Saturday of Pongpan Jirawang, a 33-year-old resident of tambon Ban Thi, who allegedly colluded with others to drop letters with distorted information on the draft charter in mailboxes around the city centre in Muang district.
The Lampang post officers detected and impounded 4,306 letters on July 12 before they had a chance to be delivered.
Police, military and local volunteers raided municipality mayor Worachart Srimai's house and office in Ban Thi district yesterday following information gained from the arrest, but found no evidence linking him to the offence.
Meanwhile, Lampang police interrogated Mr Pongpan for almost three hours yesterday.
Muang Lampang police superintendent Nikom Kruanopparat said the suspect told police he only put the letters into envelopes, and was not responsible for trying to post them.
Mr Pongpan told investigating officers he did not hatch the plan or oversee it, Pol Col Nikom said.
Police will try to track down the mastermind and other offenders who put the envelopes into the mailboxes from information given by Mr Pongpan, said Pol Col Nikom.
The officer said the arrest of Mr Pongpan followed a forensic investigation of fingerprints found on the envelopes, adding a fingerprint matched that of the suspect.
There are other fingerprints on the envelopes, and police are trying to track down their owners.
Police have also arrested 35-year-old Wisarut Kunanitisan, who was implicated in the attempt to create misleading documents related to the draft charter.
The offending material was found at Chiang Mai Tusnaporn Co -- a wood furniture manufacturer and exporter -- during a raid in Chiang Mai's Muang district on Saturday.
The company is run by Boonlert Buranupakorn, chief of the Chiang Mai Provincial Administrative Organisation, whose family is known to be closely connected with the Pheu Thai Party.
The company's staff said during the raid that the letters belonged to Mr Wisarut, who was arrested later that day.
He is facing a charge of distributing material with the intent to manipulate voters in the referendum, according to a police briefing in Chiang Mai late on Saturday night.
According to the officers, Mr Wisarut confessed to the allegation, but refused to implicate others.
Meanwhile, lists of eligible voters were found to have disappeared at a polling station in tambon Tha Mai, in Samut Sakhon's Krathum Baen district, on Saturday night.
Saifon Phungpan, Moo 5 village headman in tambon Thai Mai, said she was alerted about the disappearance of the lists by his assistants, adding there were 629 eligible voters named in the lists.
She said she ordered a search for the documents, but to no avail.
Ms Saifon said a police complaint has been lodged, and the Krathum Baen district office is working to reattach the lists to the notice board.
Meanwhile, law enforcement officers in Phatthalung are looking for the culprits behind the disappearance of the voter lists in tambon Na Kayad of Khuan Khanun district on Friday night.
Officers searched the house of a man in tambon Na Kayad who was seen drinking near the polling station that night, but have found no evidence implicating him.