Haiti has asked Washington and the UN for troops to secure its ports, airport and other strategic sites after the assassination of President Jovenel Moise opened a power vacuum in the crisis-hit Caribbean nation.
The US has already said it will send FBI and other agents to investigate the killing of Moise at his home on Wednesday.
In the wake of the slaying "we thought that mercenaries could destroy some infrastructure to create chaos ... During a conversation with the US secretary of state and the UN we made this request," elections minister Mathias Pierre told AFP on Friday.
The US State Department and Pentagon both confirmed receiving a request for "security and investigative assistance" and said they remained in contact with officials in Port-au-Prince, but did not specify whether military troops would be deployed.
The UN did not immediately respond to a request for comment. A UN diplomatic source had earlier indicated a Security Council resolution was needed to do as the Haitians had asked.
Washington had already signalled its willingness to help the Haitian investigation. White House spokeswoman Jen Psaki said senior FBI and other officials would be heading to the Caribbean as soon as possible.
The development came as questions swirled about who could have masterminded the audacious assassination, with most members of a hit squad of Colombians and Americans either dead or in custody, and no clear motive made public.
Colombia's president said the head of the national intelligence directorate and the intelligence director for the national police would travel to Haiti with Interpol to help with investigations.
Amid the uncertainty, three men are being touted as potential leaders of the nation of 11 million people, more than half of whom are under the age of 20. There is no working parliament.
After days of paralysis in the capital, Port-au-Prince saw the timid return of people to the streets, shops opening and the resumption of public transport on Friday morning - but under a pall of apprehension.
People scrambled to stockpile basic necessities at supermarkets and queued to buy propane for cooking in anticipation of more instability.
In a sign of mounting fear and desperation, hundreds of people gathered outside the US Embassy in Port-au-Prince on Friday following rumours on radio and social media that the US would be be handing out humanitarian visas.
"I don't know what's going to happen tomorrow or the day after ... I am preparing for bad days ahead," Port-au-Prince resident Marjory told AFP, as she and her husband stocked up on supplies.
Gang violence, rife in the former French colony, has also increased, with clashes between groups paralysing a major highway.
The city's airport, shuttered after the attack, appeared to have reopened, according to Flightradar data.
As the shock of the killing wore off, many in the poorest country in the Americas were demanding answers.
"Foreigners came to the country to perpetrate this crime. We, Haitians, are appalled," said a resident of the capital.
"We need to know who is behind this."
Police said a 28-member hit squad of Colombians and Americans carried out the attack, but that they were still seeking its masterminds.
Others have speculated on the possible involvement of security agents, adding to the confusion.
Moise's security team has been summoned to appear before the courts.
"The president of the Republic, Jovenel Moise, was assassinated by his security agents," former Haitian senator Steven Benoit said.
"It is not Colombians who killed him. They were contracted by the Haitian state."
One of Moise's last acts as president on Monday was to appoint a new prime minister, Ariel Henry. He had not taken office when Moise was killed.
Hours after the assassination, Mr Henry's predecessor Claude Joseph said he was in charge.
While the opposition has accused Mr Joseph of power-grabbing, the United Nations has said he had authority because Mr Henry had not been sworn in.
Late Friday, a third option was floated.
In an attempt to emerge from what it called an "institutional and political vacuum," the Senate voted on a resolution to make Senator Joseph Lambert the provisional president.
But the announcement is non-binding. While it does have some support among opposition politicians, not enough senators are currently in office to legally pass the resolution.
Haiti was already in the midst of an institutional crisis before the assassination.
Moise had not organised an election since he came to power in early 2017 and the country has had no parliament since January 2020. Moise had been ruling by decree.
Some alleged attackers, including both Americans, have been arrested. Three have been killed, and at least five are still on the run, officers said on Friday.
The US said it is aware of the arrest of US citizens but declined further comment.
Colombia on Friday said 17 Colombian ex-soldiers were thought to have been involved and will collaborate on the investigation.