Originally Published: 16 FEB 23 03:45 ET

Updated: 16 FEB 23 13:29 ET

By Aya Elamroussi and Samantha Beech, CNN

(CNN) -- The head of the US Environmental Protection Agency told CNN the agency plans to hold the train company Norfolk Southern accountable as investigations continue into the derailment of a train carrying hazardous chemicals earlier this month in a small Ohio town.

Speaking to CNN's Jason Carroll on Thursday morning in East Palestine, Ohio, EPA Administrator Michael S. Regan said the agency has full authority to use its enforcement capabilities over the crisis.

"We issued a notice of accountability to the company, and they've signed that, indicating that they will be responsible for the cleanup," Regan told CNN. "But as this investigation continues, and as new facts arise, let me just say, and be very clear, I will use the full enforcement authority of this agency, and so will the federal government, to be sure that this company is held accountable."

The interview comes two weeks after a train carrying toxic chemicals derailed in East Palestine, a town of under 5,000 people along the Ohio-Pennsylvania border. The derailment led to a blaze that lasted for days and the ordered evacuation of residents until local and state officials declared that the air and water safe enough for people to return.

State officials have repeatedly said water from the municipal system -- which is pulled from five deep wells covered by solid steel casing -- is safe to drink. However, the state's EPA encouraged residents who get water from private wells to get that water tested, the governor's office said.

Despite the assurances, a chemical odor lingered days afterward and officials estimate thousands of fish were killed by contamination washing down streams and rivers, fueling residents' concerns about water and air safety.

Hundreds of East Palestine residents attended a town hall Wednesday night to express their frustrations and mounting distrust. The train operator had agreed to attend but later pulled out of the event due to safety concerns.

Regan, the EPA administrator, was one of those frustrated by Norfolk Southern's absence.

"I was extremely disappointed that they didn't show up at the town hall meeting last night. The public deserves transparency," he said. "The public deserves to have the latest information. And so it's our job, as the federal government, to hold this company accountable, and I promise you we will."

Regan arrived in East Palestine Thursday morning to assess the ongoing response to the derailment. The agency said he will meet with city, state and federal leaders and hear directly from community members. Regan plans to discuss the EPA's 24/7 air monitoring and work being done to ensure the health and safety of the community, the EPA said.

He encouraged people with any symptoms, or pets with symptoms, to seek medical attention.

"We need for this information to get to the state health department so that we can coordinate and be sure that people are getting the help that they need."

Regan added: "We are testing for the full breadth of toxic chemicals that were on that train that was spilled. We have the capabilities to detect every single adverse impact that would result from that spill, and that's what we're doing."

Also on Thursday, four US senators representing Ohio and Pennsylvania signed letters to the EPA and the National Transportation Safety Board pushing for answers about the agency's response. The letter requests more information on their plans and ability to respond to the derailment, long-term effects on the environment and how Norfolk Southern would be held accountable.

Mayor explains actions at town hall

The town hall on Wednesday night included comments from frustrated residents unsure of their safety.

"Is it OK to still be here? Are my kids safe? Are the people safe? Is the future of this community safe?" East Palestine resident Lenny Glavan told reporters at the meeting. "We all know the severity of that question, and what's at stake. Some people think they are downplaying; some people don't think so -- let's find out."

Further spurring residents' questions about safety were crews' decision to conduct controlled detonations February 6 of some of the tanks that were carrying toxic chemicals, including vinyl chloride that has the potential to kill at high levels and increase cancer risk.

The detonation essentially released those chemicals into the air, but officials said they didn't have other viable options.

"There (were) two options: We either detonate those tanks, or they detonate themselves," Mayor Trent Conaway told a group of reporters at Wednesday's meeting. "Yes, harmful chemicals went into the air. I am truly sorry, but that is the only option we had. If we didn't do that, then they were going to blow up, and we were going to have shrapnel all across this town."

Conaway also expressed the need for assistance from the EPA.

"I need help," Conaway told reporters Wednesday night. "I have the village on my back, and I'll do whatever it takes ... to make this right. I'm not leaving, I'm not going anywhere. This is my town. I'm not going to sell my house. I'm not going to move my kids out of the school. I'm here to stay."

Train operator declines to attend due to safety concerns

Representatives of the train's operator, Norfolk Southern, planned on attending Wednesday night's meeting to provide information to residents on how they're responding to the chemical crisis. But the company backed out, citing threats against its employees.

"We have become increasingly concerned about the growing physical threat to our employees and members of the community around this event stemming from the increasing likelihood of the participation of outside parties," the company said in a release.

"We are not going anywhere," the company noted, adding it's "committed to East Palestine and will continue to respond to community concerns."

Nate Velez, who said he lives less than half a mile from where the train derailed, told CNN on Wednesday night that the company's absence from the meeting was "a slap in the face."

Velez and his family are temporarily staying in rentals away from the town. He previously told CNN that when he visited the town Monday, a chemical odor left his eyes and throat burning, and gave him a nagging headache.

"Most people did not want to go home, but they had to. So, all the people who had to go home were complaining of smells, pains in their throat, headaches, sickness," he said. "I have gone back a few times, and the smell does make you sick. It hurts your head."

Longtime resident says her home is unsafe

Jami Cozza's family has lived in East Palestine for generations near the contaminated creek, but right now she is staying at a hotel paid for by the railroad due to toxicity from the derailment.

Speaking to CNN's Don Lemon, Cozza said the railroad company told her it was safe to return home after conducting air testing. However, she insisted the railroad company run soil and water tests, and only then did a toxicologist deem her house unsafe.

"Had I not used my voice, had I not thrown a fit, I would be sitting in that house right now, when they told me that it was safe," Cozza said Thursday.

Cozza said she worried that not all residents are receiving the proper level of testing.

"My concern is how many of those kids are laying in their bed in East Palestine right now that are not safe," she said. "I absolutely do not trust them."

The railroad is providing hotel accommodations for her family and has offered to pay all of her moving expenses, according to Cozza.

"It's not about the money. It's about our house," she said.