The shooting happened around 2 p.m. in the 11600 block of Willowbrook Avenue, according to the department. It started when four deputies who were in the area responded to the C-Line platform after being alerted that a man had just been shot there.
When they reached the platform, they say the suspect jumped down onto the eastbound rail tracks, pointed a gun at them and fired.
The deputies returned fire, striking the suspect. No deputies were injured.
Investigators say the suspect had already shot and killed the other man before deputies arrived.
"The video revealed that the same suspect had walked up to this male Hispanic, we believe to be between 50 and 60 years old, and for some unknown reason shot him in the back of the head," Lt. Derrick Alfred.
The suspect was taken to a nearby hospital in unknown condition.
Service on the Green Line was temporarily impacted and passengers were being transferred to shuttle buses.