Marois arrived in Lac-Megantic hours after police said they had recovered 5 more bodies, raising the body count to 20. Workers searched through the epicenter of the explosions for the remaining 30.
Edward Burkhardt, president and CEO of U.S.-based Rail World Inc., which owns the runaway train, also was in town. He arrived Wednesday with a police escort and faced jeers from residents.
Marois had earlier faulted Burkhardt for what she said was a slow response, and called the company's chief behaviour "deplorable" and "unacceptable." She renewed some of the criticism Thursday.
"I already commented on his behaviour and the behaviour of his company yesterday. The leader of this company should have been there from the beginning," Marois said at a news conference.
Burkhardt 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. He was planning 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."
Burkhardt has blamed the engineer for failing to set the brakes properly before the unmanned train hurtled down a seven-mile (11-kilometre) incline, derailed and ignited in the centre 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.