A train carrying over 60,000 pounds of an explosive chemical mysteriously arrived empty in California two weeks after departing from Wyoming, sparking concerns about the security and safety of rail transportation. The missing cargo comprised of ammonium nitrate, a substance commonly used as fertilizer but also known as a key component in explosives that have been used in various high-profile bombings in the past.

According to Union Pacific, the rail company responsible for transporting the chemical, its investigation indicates that “criminal or malicious activity” is unlikely the cause. A representative from the Federal Railroad Administration stated that the leak may have occurred due to one of the train car’s gates not being properly closed during the thousand-mile journey, causing the chemical to seep through the rail car. The California Public Utilities Commission is also looking into the matter.