(CNN) Rumors started swirling on Tuesday that the news was coming.

Residents of West, Texas, hoped they would finally learn what happened the night of April 17, 2013, when a deadly explosion at a fertilizer plant leveled their town. The blast devastated the small town about 70 miles south of Dallas, destroying 500 homes and killing 15 people, including 12 first responders.

Lifelong West residents Mike and Janet Sulak lost their home in the blast. They had suspicions as to what caused the fire based on bits and pieces of information over the years. Maybe an electrical fire caused the explosion, they thought, something accidental, unintentional, unforeseeable.

Nothing could prepare for them for the truth.

After conducting 400 interviews and lab work on evidence, investigators determined someone started the fire on purpose, officials from the Bureau of Alcohol, Tobacco, Firearms and Explosives said.

Authorities would not comment on whether they know of a suspect. But "we're headed in the right direction," ATF Special Agent in Charge Robert Elder said. The ATF is offering up to $50,000 for information leading to the arrest of whoever set the blaze.

The Sulaks watched the news conference in their family-owned pharmacy, West Drug. It left them in complete shock.

Instead of getting answers, they have more questions.

"I'm at a loss," Mike Sulak said. "I can't imagine the intent of the person who would've set it on fire."

By the numbers 15: People killed 500: Homes destroyed 37: City blocks damaged 2.5: Miles covered with debris $2 million: Cost of investigation

'Like a nuclear bomb went off'

Families of the those killed have struggled to find answers to what happened on April 17, 2013.

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That night, a fire broke out at the West Fertilizer Co. About 20 minutes later, the plant exploded with such force it caused a magnitude-2.1 earthquake.

"It was like a nuclear bomb went off," West Mayor Tommy Muska said.

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A deafening boom echoed for miles. The blast stripped a 50-unit apartment complex of its walls and windows.

It was "massive -- just like Iraq, just like the Murrah (Federal) Building in Oklahoma City," said D.L. Wilson of the Texas Department of Public Safety.

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The blast left a crater 93 feet wide and 12 feet deep.

It also wounded another 200 people in the town of 2,800.

Tons of volatile material

The West Fertilizer Co. had warned state and local officials but not federal agencies that it had 270 tons of highly volatile ammonium nitrate on site, according to regulatory records.

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The fertilizer company had been cited by federal regulators twice since 2006.

A U.S. Chemical Safety Board investigation concluded the explosion was preventable , board chairman Rafael Moure-Eraso said on the one-year anniversary of the blast.

"It resulted from the failure of a company to take the necessary steps to avert a preventable fire and explosion and from the inability of federal, state and local regulatory agencies to identify a serious hazard and correct it," Moure-Eraso said.

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Elder, the ATF special agent, said Wednesday that the owner of the plant has been cooperating with authorities.

Victims as heroes

The blast was so catastrophic, it damaged or destroyed houses across 37 blocks, including the home where the Sulaks raised their family.

They count themselves among the lucky ones. They were out to dinner in nearby Waco when the blast occurred. It damaged their home beyond repair but miraculously, their family pharmacy, West Drug, survived.

After more than two years of living with family they moved back to the site of their old home in December. Mike Sulak estimates that donations of money, materials and manpower from across the state saved them $15,000 in rebuilding costs.

"Words to describe it are tough. It brought us all together; we have shared a common experience."

Rebuilding continues elsewhere in town, Janet Sulak said. The nursing home has opened in a new location. This fall the Sulaks' grandchildren will no longer have to attend school in trailers, after the new high school opens. The Sokol gymnasium, a fitness hall with roots in the town's Czech heritage, is open again.

News like this reopens old wounds, she said. Many in town wish it would end.

"It keeps coming up all the time," she said. "Just let it rest, already."

Even if it was arson, part of her still believes it was an accident. Even if someone set the fire, surely it was never the intention for it to spread like it did.

"I don't want to blame anyone for it," she said. "But it happened and I guess we will have to accept whatever the board comes up with and move on from there."