In an e-mail interview, Royce said he has sent a letter to Secretary of State noting his concern for an ongoing culture of impunity.
Cambodia has around 200 government-provided land concessions, totally 2 million hectares.
The men had been forced to work long hours for little pay aboard the boat before escaping to Indonesia.
Prime Minister Hun Sen announced last week he wanted the men arrested.
Some of Wednesday’s female protesters tore off their shirts as they faced off with riot police carrying batons and shields.
Hun Sen said Tuesday he would take back land concessions if the violence continues.
Nearly 80 land cases involved government land concessions that affected more than 30,000 families.
Sia Phearum told “Hello VOA” people are being forced off land they earned through “backbreaking” work.
Nearly 400 families were forcibly evicted from the Borei Keila site this month.
Provincial Police Chief Chhoung Seanghak said police are working the case and focusing on the two guards.
The Documentation Center has collected nearly 1 million documents on the Khmer Rouge since 1995.
Cambodia is pushing for a seat on the council, UN Security Council, for 2013 and 2014; voting will be held later this year.