Four years after the West Fertilizer Plant Explosion, homes and a school are rebuilt, but victims' families do not know who intentionally caused the fire that spurred the blast that killed 15 people, injured more than 200 and leveled hundreds of homes.

The Federal Bureau of Alcohol, Tobacco, Firearms and Explosives is still offering a $50,000 reward for tips to find the person who set the fire, which led to the explosion -- an explosion so powerful that its force was equal to that of a magnitude 2.1 earthquake.

Federal investigators have interviewed hundreds of people in the years since, but no arrests have ever been announced. The U.S. Chemical Safety and Hazard Investigation Board, which published a final report on the explosion in 2016, called it one of the most destructive tragedies its staff ever investigated.

Twelve first responders were among the dead.

Emergency crews first received word of the fire at 7:30 p.m. on April 17, 2013. The massive explosion happened approximately 20 minutes later.

To date, the ATF has spent more than $2 million on the investigation, which the bureau said was one of its largest in history.

The rebuilding process has been slow but steady. While signs of devastation are impossible to miss near the blast site, a new nursing home and public park have both appeared in the four years since the explosion. West Middle School and West High School, which were both destroyed in 2013, reopened together in 2016 in a new state-of-the-art facility. The $50 million facility was funded through a mix of insurance money, along with funds from the Texas Education Agency and FEMA.

A 30-minute memorial ceremony is scheduled for 7:30 p.m. Monday at St. Mary's Catholic Church of the Assumption in West.