In Minnesota, an activist says about 2,000 of the local Cambodian community have registered to vote, slightly more than the 2012 election.
Observers say advances in technology had made producing drugs easier and harder to detect.
Daniel Russel, assistant secretary of state for East Asian and Pacific Affairs, on Thursday met officials from the government and opposition as well as civil society leaders, expressing hopes for “free and fair” elections in 2018.
Prior to his killing Kem Ley had appeared on several radio programs speaking about high-level corruption in the government.
Ethnic group is known for interest in politics, and also for generational divide in political preferences
Phil Robertson of Human Rights Watch said the situation is unlikely to improve unless Western countries focus more on human rights and democracy in their engagement with Southeast Asia.
Political analysts said relying on international pressure to improve the human rights situation would not provide long-term solutions.
Her arrival came a day after security forces beat rights workers and land activists during a demonstration in the capital against forced evictions.
He was also fined 4 million riel (about $1,000) for comments he made last year about alleged Vietnamese encroachment on Cambodian territory.
At least one human rights worker and a land rights activist were attacked while celebrating World Habitat Day on Monday.
On a mission to Preah Vihear province in March to meet with local indigenous groups, her car was blocked by security forces.
Kem Ley was gunned down on July 10 at a gas station in the capital. One suspect has since been arrested and charged with pre-meditated murder.