(CNN) — Kentucky residents who evacuated after more than a dozen freight train cars derailed, spilling molten sulfur, can now return home safely as the fire has been extinguished and the air monitored, train operator CSX said Thursday.

At least 16 cars were involved in the Thanksgiving eve derailment north of Livingston, “including two molten sulphur cars that have been breached and have lost some of their contents which is on fire,” CSX, said in statement Wednesday. Livingston is a small city roughly 60 miles south of Lexington. Authorities had encouraged residents to evacuate.

“Specialized equipment has been deployed to conduct air monitoring in the area and local authorities have determined it safe for residents to return to their homes,” CSX said. The focus at the scene has shifted to removing the derailed cars and recovering product on the ground.

“The cause of the incident is under investigation,” the railway company said in a statement. “CSX is still supplying food, lodging and other necessities to affected community members.”

When molten sulfur burns, it releases sulfur dioxide, CSX said. Sulfur dioxide is a colorless gas with a strong scent. Depending on the level of exposure, it can cause irritation to the eyes, nose and throat – while exposure to its liquid form could cause frostbite, according to the US Centers for Disease Control and Prevention.

Earlier Thursday, Kentucky Emergency Management spokesperson Jordan Yuodis told CNN 50% of the fire had been contained but residents who evacuated were not being allowed to return home.

“Due to the train derailment, many families in Livingston … will be displaced for Thanksgiving. Please think about them and pray for a resolution that gets them back in their homes,” Kentucky Gov Andy Beshear said on Facebook Thursday morning.

The Environmental Protection Agency was monitoring sulfur dioxide levels in the county, EPA on-site coordinator Matthew Huyser told reporters Thursday. The derailment caused increased levels in the immediate area, but those levels have decreased as the fire is being extinguished, he said.

The EPA’s website said short-term exposure to sulfur dioxide can harm the human respiratory system and make breathing difficult, especially for children and people with asthma.

Huyser did not provide sulfur dioxide levels measured overnight but said the goal is to reach levels of zero. “It appears that the firefighting efforts have been successful in reducing and quite eliminating the hazards that are being measured,” he said.

CSX said it will provide food, shelter and Thanksgiving dinner for the displaced families.

Linda Todd said she evacuated her Livingston home Wednesday after she was warned of safety concerns.

“I was freaking out because I said, ‘We are cooking, we got turkeys in the ovens. We can’t leave.’ They were like, ‘You have to go, it is a bad situation. You have to go,’” Todd told CNN affiliate WYMT.

Beshear declared a state of emergency in response to the derailment, and his office said the state’s Emergency Operations Center has been activated.

“By issuing a state of emergency, we are ensuring that every state resource is available to help keep our families safe,” Beshear said.

The derailment happened just before 2:30 p.m. Wednesday, CSX said. The train derailed between Mullins Station and Livingston, the Rockcastle County sheriff told CNN affiliate WKYT. One member of the two-person crew was treated at the scene for minor injuries, the train company said.

Two of the other cars that derailed were carrying magnesium hydroxide, but there was no indication they were breached, CSX said Thursday. The other impacted cars were carrying non-hazardous products or were empty.

The crash led officials to shut down US 25 in both directions from the Laurel County line to Calloway Branch Road, and it’s unclear when it will reopen, the Kentucky Transportation Cabinet for District 8 said in a social media post Wednesday evening.

Correction: A previous version of this report misstated the agency Jordan Yuodis represents. He is the public information officer for Kentucky Emergency Management.