Authorities discovered letters containing allegedly distorted information on the draft charter during raids on six locations linked to the chief of the Chiang Mai Provincial Administrative Organisation on Saturday.
More than 200 military, police and administrative officers took part in the operation.
The raided locations were Chiang Mai Tusnaporn Co -- a wood furniture manufacturer and exporter run by Boonlert Buranupakorn, president of the Chiang Mai PAO, in tambon Tha Sala of Muang district -- a house opposite the firm and four places on Chiang Mai-San Kamphaeng Road.
Mr Boonlert's family have been closely connected with the Pheu Thai Party.
Officers discovered envelopes bearing the addresses of residents and letters indicating the disadvantages of the draft charter. The envelopes, copy machines and related equipment were seized for examination.
Police said the letters contained distorted information that claimed the draft charter would cancel the government's 30-baht healthcare scheme, the elderly's monthly payments and free schooling.
Tusnaporn staff members told officers that the items belonged to Wisarut Kananitisan, a 35-year-old man who is close to a prominent politician in Chiang Mai.
Officers found no trace of Mr Wisarut, who lives in the house opposite the firm.
A jacket, a crash helmet, shoes and a motorcycle found in the house are similar to those belonging to a man earlier seen on CCTV footage dropping letters in mailboxes across Chiang Mai, police said.
Also raided were an office and house of Kachen Jeakkajon, mayor of tambon Chang Phuak municipality, where officers reportedly found some evidence linked to the letters. Mr Kachen is the husband of Mr Boonlert's niece.
The raids followed the arrest of 63-year-old Samart Kwanchai, who allegedly handed out leaflets containing distorted charter information at a shopping mall on Friday evening.
According to Mr Boonlert's associates, the Chiang Mai PAO chief flew out from Thailand on Tuesday. He transferred in Japan and flew on to Los Angeles in the US. He is due to return to Chiang Mai on Aug 2.
Pheu Thai officials contacted by the Bangkok Post Sunday declined to comment yesterday.
Meanwhile, two high-school students have confessed to tearing down three lists of eligible voters for the charter referendum attached to a notice board at a polling station in Rayong's Klaeng district.
They were among four students playing in the area on Friday. The four were questioned by police to determine whether they were hired by someone to rip the documents. Klaeng district chief Wittaya Chapanon also attended the session.
The two students told police there was no one behind their acts, adding they thought these were old documents. They were sent to Rayong juvenile court to face prosecution.
Two eight-year-old girls who admitted tearing down voter lists attached to a noticeboard at a school in tambon Salok Bat of Kamphaeng Phet's Khanu Woralaksaburi district last Saturday have been charged with obstructing the referendum process, destroying official documents and destroying common public property, according to provincial police chief Damrong Petpong.
They will not face jail time as Thai law exempts anyone under the age of 10 from criminal punishment, he said, adding that police were still duty-bound to file the charges.
Authorities have stepped up efforts to ensure security before the referendum.
Deputy police chief Pol Gen Srivara Ransibrahmanakul has instructed police to deploy force to maintain peace.
He also asked police to coordinate with the army and administrative officers to monitor any groups that could stage movements before the referendum.
Seeking evidence: Documents are examined at Chiang Mai Tusnaporn Co.
Official response: A briefing is held in Chiang Mai over the arrests.