A group of Arkansas inmates is asking the U.S. Supreme Court to block upcoming executions in the state, citing concerns with one of the lethal injection drugs. 

The request was filed on Tuesday by eight inmates who the Arkansas governor scheduled execution dates for this month. 

A ninth death row inmate who doesn't have a scheduled execution date also signed on to the request. 

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Michael Radelet is an expert when it comes to the death penalty. He's a sociology professor at the University of Colorado who has written seven books on the topic. 

According to his research, there have been 103 executions in our state's history. The last one was in Cañon City in 1997. There are three people currently on death row in the state. Radelet says in Colorado, the death penalty comes down to three things:

"In Colorado, there are three factors that drive the death penalty, one is politics, two is politics and three is politics, and with the expense of the death penalty, every time we seek the death penalty it costs upwards of 10 to $20 million," Radelet said. 

Radelet says it's important to understand not only who gets the death penalty, but who doesn't and that cases vary by counties.

Two death penalty cases happened within the last couple of years. The theater shooter was up against a death sentence and so was Dexter Lewis, who stabbed five people in a Denver bar. Both crimes took place in 2012 and both were sentenced to prison. 

The Associated Press contributed to this report.