A delegation of senior US security and justice officials on Sunday arrived in Haiti to assess the security situation.
The team will also meet three Haitian politicians. each of whom is claiming to be the country's legitimate leader.
After the attack, Haiti's authorities asked the US and the UN to send troops to the country to protect key infrastructure.
President Joe Biden's administration initially rejected the request - but has now decided to have a closer look at the situation.
Mr Moïse had been president of Haiti, the poorest nation in the Americas, since 2017. His time in office was rocky as he faced accusations of corruption and there were widespread demonstrations in the capital and other cities earlier this year.
Parliamentary elections should have been held in October 2019 but disputes have delayed them, meaning Mr Moïse had been ruling by decree. He had planned to hold a referendum on the proposed constitutional changes this September.
In February this year, on the day the opposition wanted him to leave office, Mr Moïse said an attempt to kill him and overthrow the government had been foiled. It is still unclear who organised last week's attack and with what motive. A number of questions remain unanswered, including how the alleged assassins were able to enter the property. Mr Moïse's bodyguards are due to be questioned later this week.
One prominent opposition figure has openly expressed scepticism over the current version of events. Former Haitian senator Steven Benoit told local station Magik9 radio on Friday it was "not Colombians who killed him", but did not provide evidence to back up his claims.
Haitian police have said the majority of the mercenaries was Colombian, while two were joint US nationals.
