Nong Khai Customs chief Nimit Saeng-ampai said on Wednesday the suspects’ Toyota Land Cruiser had been in and out of Thailand at least nine times.
The same vehicle is believed to have been used to transport cash each time, possibly to avoid detection by Lao authorities, he said.
Tuesday’s stash was allegedly found hidden inside milk boxes and in the engine compartment.
Investigators from the Office of the Narcotics Control Board and the Anti-Money Laundering Office joined Customs agents in examining the case of arrested siblings Suebanh Tiasili, 30, and Khumborn Tiasyly, 24.
The duo applied for bail release, offering to leave all the confiscated cash to the Thai state, Nimit said.
They were released on bail, but the cash and vehicle were seized, along with their passports, pending further investigation.
Nimit said officials were trying to verify the brothers’ claim that their family was in the currency-exchange business and they’d collected US$2.8 million worth of Thai banknotes at various banks in Nong Khai to take into Laos.
He noted that the baht was currently stronger against the US dollar than the Lao kip, so it was unlikely the money exchange was aimed at turning a profit.
Thai law prohibits amounts exceeding Bt450,000 to be carried outside the country without the relevant officials being notified.