CHICAGO (CBS) — CTA officials determined “human error” was the cause of the June 6 CTA Green Line derailment, according to a release from the CTA.

On June 6, the front car of a southbound CTA Green Line train approaching 47th Street slid off the rails, leaving 30 passengers stranded more than 20 feet above the ground on the elevated track in the in the Bronzeville neighborhood.

The seven passengers transported to the hospital sustained only minor injuries.

A CTA spokesperson said the human error was attributed to three employees — two CTA signal maintainers and a CTA rail operator.

“Two signal maintainers were working along the tracks north of the 47th Green Line Station and had previously moved a switch—a manually controlled lever which controls a train’s routing along the track—from its normal position to a reverse position,” the CTA spokesperson said in a press release.

Officials said the switch should have been “set to accommodate southbound trains, but instead the switch was set so that the tracks were not aligned, causing the derailment.”

According to the spokesperson, the two CTA signal maintainers received 10-day suspensions without pay, have been placed on six months’ probation and were given written warnings.

CTA officials said the CTA operator’s role in this incident is still under review. The operator has been off work since the incident.

“Safety is CTA’s number one priority and we remain focused on ways to further improve our already strong safety record,” CTA officials said.