Maltese investigative journalist Daphne Caruana Galizia, 53, was killed Monday as she drove away from her home in the town of Mosta, on Malta's main island, said Prime Minister Joseph Muscat, according to The Associated Press.
U.S. Department of State spokesperson Heather Nauert condemned Galizia's death as "appalling violence in the strongest terms possible" in a press briefing Tuesday.
Nauert called for a "thorough, transparent" investigation, saying that the FBI is providing "specific assistance as needed" at the request of Maltese authorities.
Muscat described Galizia's death as a "barbaric attack" and a "political murder," as well as an assault on freedom of expression.
Galizia was "was one of my harshest critics, on a political and personal level," Muscat said. Galizia wrote that Muscat's wife, along with the country's energy minister and the government's chief of staff, had offshore holdings in Panama to receive money from Azerbaijan, according to the AP. Muscat and his wife denied that they had companies in Panama.
Thousands of people gathered in Sliema Monday night for a candlelight vigil in tribute to Galizia.
Herman Grech, the online editor of the Times of Malta, called her a "dedicated" journalist, adding "no one could stop her."
"She had this uncompromising state of mind. I didn't always agree with her methods. She got this niche of fighting corruption and I don't know any journalist in Malta who will be able to take back her investigations," Grech told ABC News.
The slain journalist had been a columnist for The Malta Independent for more than 20 years and also wrote a blog called the "Running Commentary." She is survived by her husband and three sons.
ABC News' Conor Finnegan and Ben Gittleson and The Associated Press contributed to this report.