Cambodia and the US will mark 60 years of diplomatic relations in July.
Pen Sovann was the prime minister under the first government put in place by the Vietnamese.
Rapp is on a two-day visit to Phnom Penh, where he met with government officials, NGOs, donor representatives and tribunal officials.
International and Cambodian prosecutors found themselves at odds last year over whether to indict five more Khmer Rouge leaders.
Hor Namhong has said he was a victim within the Boeung Trabek camp, where several inmates died.
Civil parties without lawyers lose the right to appeal if they are dismissed by investigating judges.
Jailed Khmer Rouge leader Khieu Samphan has been hospitalized since last week, tribunal and hospital officials said Tuesday.
The March 18 letter questions Hor Namhong’s role in supervising the Boeung Trabek camp in 1978.
Duch’s trial, which ended in November 2009, was a test case for the court.
Donor countries have approved an $85 million budget for the next two years of the court.
Judges at the Khmer Rouge tribunal are prepared to begin investigating additional suspects beyond those already in custody.
An estimated 5,000 people died at this commune alone, 300 of them in Kandal village.