The dense jungles of southern Thailand and northern Malaysia have been a major stop-off point for smugglers bringing people to Southeast Asia by boat from Myanmar, most of them Rohingya Muslims who say they are fleeing persecution, and Bangladesh.
In January, the government had said it would set up a panel to inquire into claims that authorities mishandled an investigation into 139 mass graves and more than 12 campsites suspected to have been run by migrant-smuggling gangs.
Three police from a jungle infantry unit described finding the first of the campsites in January 2015, during a patrol in a heavily-forested region on the Malaysian side of the border.
The officials found an area filled with tents and structures made of wooden sticks, some as high as two stories, said one of them, Mat Ten.
"These makeshift houses were surrounded by barbed wire and there were people living inside," he told the panel.
The men's testimony confirmed media reports and rights groups' statements that authorities had known about the camps four months before going public in May 2015.
In a report last month, the Malaysian Human Rights Commission and rights group Fortify Rights said authorities had destroyed one of the camps a day after its discovery, wiping out evidence that could have aided police investigations.
The human rights commission was set up by Malaysia's parliament but the government is not bound by its findings.
Mat Ten said he returned with a team of ten police to survey the site the day after the first visit, but they were interrupted after a woman they believed to be acting as a lookout alerted others at the camp, forcing the team to flee.
Six Rohingya and Bangladeshi migrants who were left behind were detained and handed over to immigration authorities, said Mohamad Mossadique Azni, the officer who led the raiding team.
The discovery of similar camps and graves on the Thai side of the border triggered a regional crisis in 2015.
A Thai crackdown on the camps prompted traffickers to abandon thousands of migrants in overloaded boats in the Bay of Bengal and Andaman Sea.The hearings continue on Thursday.