It’s unclear how Larry Dalzell and Randi Vandekieft became trapped in the silo in Volga, South Dakota. Their bodies have been recovered and police are investigating.

Two workers who became trapped inside an 80-foot farm silo that they were repairing in eastern South Dakota have died, presumably killed by toxic gas.

The accident occurred shortly before noon on Saturday at a rural farm in the small town of Volga, the Brookings County Sheriff’s Office said in a Facebook post on Monday. The victims were working on a damaged roof above a nearly full 80-foot-tall silo.

Larry Dalzell, a 51-year-old from central Tennessee, and Randi Vandekieft, a 41-year-old from central Georgia, were both recovered from the silo and later pronounced dead. The sheriff’s office said the two were “believed to have been overtaken by toxic gas.”

It’s unclear how the two became trapped in the silo. The Brookings County Sheriff’s Office is investigating the deaths.

What is Heartland Tank?

Dalzell and Vandekieft were both working for Oklahoma City-based engineering firm Heartland Tank, according to the the sheriff’s office.

Formed in 2007, the company provides liquid storage, secondary containment alternatives and inspections through “innovative leadership and exceptional service with quality, integrity, and dedication,” according to its website.

“Heartland Tank Services is committed to safety and believes proper training and retention of employees is essential to the business,” the company’s website states. “All employees within each company have the necessary certifications for crane operation, welding, confined spaces, scaffolding, and API-653 inspections.”

USA TODAY has reached out to Heartland Tank for comment.