The National Transportation Security Board (NTSB) issued its first preliminary report Thursday on the Feb. 3 derailment of a train carrying hazardous chemicals in East Palestine, Ohio.

While the report seemingly faults an overheated bearing for the derailment, the NTSB investigation is ongoing, and a number of question marks remain.

Here are five remaining questions about the train derailment:

In the preliminary report, the NTSB said it has decontaminated the train cars that contained hazardous material, which included vinyl chloride. Exposure to the substance, which is used for production of plastics, has been linked to higher rates of liver and lung cancer.

NTSB officials said they have removed and examined the top fittings from the cars that contained the substance, and that the fittings will be sent to Texas for further testing.

Public safety advocates have frequently pointed to the Trump administration’s delay of a 2015 rule, vocally opposed by railroad companies, that would require the use of more modern electronically-controlled pneumatic brakes.

Although the Transportation Department has said the rule would not have prevented the East Palestine disaster, both Transportation Secretary Pete Buttigieg and President Biden have pointed the finger at the railroad lobby for deregulation and its potential role in the accident.

The NTSB noted in its report that it is not involved in “air monitoring, testing of water quality, environmental remediation, or evacuation orders” relating to the disaster. Much of this falls under the purview of the Environmental Protection Agency (EPA), which has taken over the response. EPA Administrator Michael Regan said last week that the agency will require Norfolk Southern, which operates the railroad, to assume financial responsibility for all cleanup efforts.

While the EPA and Gov. Mike DeWine (R) have said water in the area has tested as safe to drink, they have recommended drinking bottled water, and state officials have reported at least 3,500 fish have died in surrounding waterways

DeWine ordered an evacuation of the derailment area days after the initial derailment. Shortly after, concerned about a potential explosion due to the presence of flammable compounds, first responders conducted a controlled burn. In its preliminary report, the NTSB listed “venting and burning of the vinyl chloride” as an ongoing subject of investigation. A number of lawsuits have been filed in the meantime alleging the controlled burn exacerbated hazards, including a complaint by local injury firm Morgan & Morgan claiming it has contributed to locals’ health problems.

Many of the potential risks associated with environmental disasters, from cancer to developmental issues, can take months or years to become apparent, and will require ongoing investigations by both the NTSB and the EPA.

Regan and Ohio Sen. Sherrod Brown (D) have urged residents to continue testing their homes’ air quality, while Brown has also said he is working to ensure Norfolk Southern does not manipulate residents into waiving their right to sue.

Another unknown is any possible risks to the surrounding areas, not just other parts of Ohio but the other side of the state line as well. Pennsylvania Gov. Josh Shapiro (D) told NPR this week that Norfolk Southern’s response has “given the middle finger to the good people of Pennsylvania and Ohio” and that testing of western Pennsylvania water is ongoing as well. “We’ve seen no concerning readings yet, but we’re going to continue to test for months and months and months, if not years,” he said.