A group of unidentified individuals raided Moise's private residence in Haiti's capital, Port-au-Prince, at about 1 a.m. local time, according to a statement from Haitian interim Prime Minister Claude Joseph. They gunned down the 53-year-old head of state and wounded his wife, Martine Moise, who was transferred to the U.S. for treatment.
Joseph, who condemned what he called a "hateful, inhumane and barbaric act," declared a "state of siege" and said that the country's national police force and military had the situation under control, with himself "in charge" now.
Late Wednesday, Haiti's communications secretary said in a tweet that police have arrested the "presumed assassins," but Frantz Exantus did not provide further details about Wednesday's slaying or say how many suspects had been arrested.
Haiti's police chief said later Wednesday that four of the suspected killers were fatally shot by police and two others arrested in an apparent hostage-taking situation, the Associated Press reported. Leon Charles said that three police officers held hostage were freed.
Reactions have been pouring in from around the world condemning the assassination, including from President Joe Biden, who called the situation "very worrisome."
Reeling from the coronavirus pandemic, Haiti has also been in the midst of a constitutional crisis as Moise and opposition leaders disputed the end of his five-year presidential term and legislative elections remained interminably delayed.
The country's line of succession is unclear, especially given its recent political turmoil, but Joseph suggested he would only take command temporarily and called for presidential elections "to move the country forward," he told the Associated Press.
"All the ministers and I have been working since the news broke and we want to assure you we will bring the killers of the president to justice," he said during a televised speech to the nation after chairing a meeting of Moise's cabinet.
"Please stay calm and let the authorities do their work," he urged the Haitian people. "We don't want the country to plunge into chaos. This is a very sad day for our nation and for our people."
U.S. Secretary of State Antony Blinken spoke to Joseph by phone Wednesday afternoon -- the highest level contact between the U.S. and Haitian governments since the assassination. In a boost to the interim leader, Blinken "reiterated" to Joseph that the U.S. would work with the government of Haiti to support the Haitian people and "democratic governance, peace, and security," his spokesperson Ned Price said.
The assailants, who remain at large, were "well-trained commandos" who were speaking Spanish and English and most likely came from outside Haiti, according to Bocchit Edmond, Haiti's ambassador to the U.S. The group was "highly trained and heavily armed," according to Joseph, who called for an urgent United Nations Security Council meeting and an international investigation into the attack.
Edmond said the Haitian government had video evidence of the group speaking Spanish. He also said they claimed to be agents from the U.S. Drug Enforcement Administration, which Edmond and Price both rejected.
"These reports are absolutely false. The United States condemns this heinous act," Price said during a briefing Wednesday.
The Haitian envoy also urged Washington to provide security assistance, not just for the immediate investigation, but also to boost Haitian security forces against armed gangs and a porous border. But Price told reporters Wednesday afternoon that the U.S. -- while ready to help -- had not yet received a formal request for assistance.
Martine Moise is in stable but critical condition, according to Edmond, and she was flown to Fort Lauderdale Executive Airport by Trinity Air Ambulance Wednesday afternoon for treatment at Miami's Jackson Memorial Hospital, according to ABC News local affiliate WPLG.
The streets of Haiti's capital Port-au-Prince were largely deserted Wednesday, as police patrolled the city streets. Toussaint Louverture International Airport was closed in the wake of the assassination, preventing the U.S. ambassador to Haiti from returning Wednesday after previously scheduled meetings in Washington.
The U.S. Embassy in Port-au-Prince was also closed Wednesday, including for consular services, it said in a security alert. The embassy also said it is restricting its American staff to its compounds "until further notice" and urged members of the public to avoid unnecessary travel to the area.
U.S. officials are " still gathering information" on the deadly attack, according to White House press secretary Jen Psaki, offering U.S. assistance for Haitian authorities' investigation. But Price deferred questions about possible motives or suspects to Haitian authorities, saying they would take the lead.
Biden condemned "this heinous act" in a White House statement earlier in the day, adding his "sincere wishes for First Lady Moïse's recovery."
"We stand ready to assist as we continue to work for a safe and secure Haiti," the statement added.
Haiti has been in a state of chaos for months now, with frequent gunfire and street skirmishes between armed groups, political demonstrations and strikes for and against Moise, and a coronavirus wave never brought under control. Cases of the coronavirus were as high last month as they were one year ago, and the country has yet to distribute a single vaccine dose or receive any shipments from COVAX, the international program to provide vaccines to low- and middle-income countries, according to the World Health Organization.
That's in part because of the governing crisis roiling Haiti. The country's political opposition had argued that Moise's five-year presidential term ended this February -- five years after his election victory, but four years after he took office -- while he said he had one more year left because the disputed 2016 election delayed his inauguration until 2017.
Moise had been governing by decree since January 2020, after the country failed to hold legislative elections and the legislature's mandate expired. Opposition leaders accused him of wanting to return Haiti to a dictatorship.
Earlier this year, Moise ordered the retirement of three Supreme Court judges and the arrest of nearly two dozen people, including prominent officials, who he alleged were plotting a coup. Violent protests against Moise erupted, prompting the president to declare a state of emergency in parts of the country in March.
Moise had told a Spanish language newspaper in January that he feared people wanted to kill him, according to Edmond. But the ambassador added there was no information or advanced warning leading up to Wednesday's attack.
The political instability in addition to economic woes and escalating gang violence have undermined efforts to rebuild Haiti from a devastating earthquake in 2010 and Hurricane Matthew in 2016.
While the Biden administration backed Moise's claim to have one more year in office, it had grown increasingly vocal in its opposition to his "one-man rule," in the words of the top U.S. diplomat for the Western Hemisphere, including governing by decrees and refusing to hold those legislative elections.
The State Department backed Joseph's calls for elections before the end of this year, with Price calling parliamentary and presidential elections "the democratic path towards ending Haiti's irregular and prolonged rule by decree and restoring its parliament."
While the White House has said it will provide Haiti some of the initial 80 million COVID-19 vaccines it has promised to share overseas, it has yet to announce when it will do so -- with the worsening security situation now making it that much harder.
ABC News' Christine Theodorou, Molly Nagle and Sarah Kolinovsky contributed to this report.