The four police officials were arrested for corruption in connection with drug trafficking.
Hun Sen was responding to concerns raised by the US lawmakers as the refugee center nears its closing date, Feb. 15.
The rangers work for the Ministry of Environment, supported by Fauna and Flora International.
Yoeung Baloung, a former police chief of Ratanakkiri province, was supposed to be in jail on charges related to illegal logging.
Officials would like to see 87 million visitors traveling across Southeast Asia by 2015.
The decision overruled a ruling by the Phnom Penh Municipal Court last week to hold five of seven Thais arrested in December.
In 2010, Cambodia produced a milled rice surplus of nearly 2.5 million tons.
The four men were charged under Article 38 of the 1992 penal code and face up to seven years in prison if convicted.
Protesters had prepared a letter for the embassy, but embassy officials would not accept it.
Rapp, who staid in Cambodia Thursday through Sunday, said his visit had convinced him the investigations will go forward.
The recent midterm elections in US Congress should serve as a good model for Cambodians on how power can be shifted smoothly.
Government ministry officials have until March to add up their worth and send it to anti-corruption authorities.