US officials have determined that an explosion at a Texas fertiliserplant was caused by arson.

Special Agent in Charge Rob Elder said the bureau of Alcohol, Firearms, and Tobacco interviewed more than 400 people in determining the cause of the April 2013 blast in the small town of West - population 2,800. The explosion killed 15 people - 12 of whom were first responders - injured almost 200, and destroyed numerous nearby homes.

“The fire has been ruled as incendiary, a criminal act,” Mr Elder said, confirming that "all accidental and natural causes" of the explosion had been ruled out. “It is our highest priority to give victims of this tragedy an accurate explanation of what happened that day.”

No suspects have been arrested, but the ATF announced that they are offering a reward of $50,000 (£34,554) for any information leading to an arrest. Authorities would not comment on whether or not they had a suspect in mind, but he did say that investigators were "on the right track."

The explosion occurred 20 minutes after a fire broke out on the West Fertilizer Co premises. Video depicting the blast shows a shockwave sent off from the plant, causing a tremor in the town equivalent of a 2.1 magnitude earthquake. West mayor Tommy Muska compared the blast to a nuclear explosion.

The US Chemical Safety and Hazard Investigation Board called the plant explosion “one of the most destructive incidents” they had ever investigated. According to Mr Elder, the explosion left a crater that was 93 feet wide and 12 feet deep. The board's investigation concluded that the explosion had been "preventable".

In the days after the explosion, which happened the same week as the deadly Boston Marathon bombings,President Barack Obama, Texas Sen John Cornyn, and then-Governor Rick Perry attended a memorial to honour the lives of the people who died in the blast, as well as console a shocked community.