He engaged in a gun battle which left one police officer and two civilians dead but ultimately surrendered inside the building after a five-hour siege.
The full extent of casualties was murky in the immediate aftermath of the gunman's capture.
State Attorney General Cynthia Coffman posted a message on her official Twitter account a short time later saying two people had been killed, but that post was quickly deleted and replaced with another lamenting a "tragic loss of life."
Buckley put the number of people taken to area hospitals at 11, including five officers, but it was not immediately clear whether that tally included the fatality reported by the police union.
"We did get officers inside the building. They were able to shout to the suspect and make communication with him and at that point they were able to get him to surrender and he was taken into custody," Buckley said.
After the arrest, Buckley said it would take hours more, and perhaps days, for investigators to fully process the crime scene.
Police swarmed the area around the building after an emergency call reporting shots fired at about 11.30am local time (6.30pm GMT), and officers ultimately confronted the suspect inside the building, Buckley said.
The FBI and agents from the US Bureau of Alcohol, Tobacco, Firearms and Explosives were assisting local law enforcement investigators.
The Planned Parenthood center provides abortions, screening for sexually transmitted diseases, pregnancy testing and other services, according to its website.
"We don't yet know the full circumstances and motives behind this criminal action," Vicki Cowart, president and CEO of Planned Parenthood Rocky Mountains, said in a statement.