Police said that 20 bodies had been recovered, so far, and 30 people remained missing and were presumed dead.
"I already commented on his behavior and the behavior of his company yesterday. The leader of this company should have been there from the beginning," Marois said at a news conference.
Prior to arriving in Lac-Megantic, Marois had faulted the train company's response calling the company's chief "deplorable" and "unacceptable," for waiting four days before coming to the town.
Edward Burkhardt, president and CEO of U.S.-based Rail World Inc., — said he had delayed his visit in order to deal with the crisis from his office in Chicago, saying he was better able to communicate from there with insurers and officials in different places.
Burkhardt arrived Wednesday with a police escort and faced jeers from residents. He was expected to meet with residents and the mayor Thursday.
"I understand the extreme anger," he said. "We owe an abject apology to the people in this town."
On Wednesday, Burkhardt blamed the engineer for failing to set the brakes properly before the unmanned train hurtled down a seven-mile (11-kilometer) incline, derailed and ignited in the center of Lac-Megantic early Saturday. All but one of its 73 cars was carrying oil, and at least five exploded.
Burkhardt said the train's engineer had been suspended without pay and was under "police control."
Investigators also had spoken with Burkhardt during his visit, said a police official, Sgt. Benoit Richard. He did not elaborate.
Until Wednesday, the railway company had defended its employees' actions, but that changed abruptly as Burkhardt singled out the engineer.
"We think he applied some hand brakes, but the question is, did he apply enough of them?" Burkhardt said. "He said he applied 11 hand brakes. We think that's not true. Initially we believed him, but now we don't."
Burkhardt did not name the engineer, though the company had previously identified the employee as Tom Harding of Quebec. Harding has not spoken publicly since the crash.
"He's not in jail, but police have talked about prosecuting him," Burkhardt said. "I understand exactly why the police are considering criminal charges ... If that's the case, let the chips fall where they may."
Investigators are also looking at a fire on the same train just hours before the disaster. A fire official has said the train's power was shut down as standard operating procedure, meaning the train's air brakes would have been disabled. In that case, hand brakes on individual train cars would have been needed.
The derailment is Canada's worst railway disaster since a train plunged into a Quebec river in 1864, killing 99.
The crash has raised questions about the rapidly growing use of rail to transport oil in North America, especially in the booming North Dakota oil fields and Alberta oil sands far from the sea.
Associated Press writers David Crary in Lac-Megantic and Charmaine Noronha in Toronto contributed to this report.
Copyright 2013 The Associated Press.