Robert Lewis Dear, 57, is believed to have entered the Planned Parenthood clinic with a high-powered rifle on Friday and fire on police and civilians outside the building.
Dear allegedly shot and killed three people, including a police officer, and wounded five others, Colorado officials said.
The motive for the rampage however is unclear.
When questioned after his arrest Dear mentioned “no more baby parts” in reference to the abortions Planned Parenthood provides, NBC News reported, citing law enforcement officials.
In his rambling interview Dear also mentioned President Barack Obama, so the sources told NBC that it was unclear whether he targeted the clinic because of abortions.
Police have said little about Dear, who is being held without bail and will appear in court for an arraignment on Monday.
US media described Dear as a troubled loner who over the years had several minor brushes with the law.
In 1997, Dear’s then wife, Pamela Ross, once called the police to accuse him of domestic violence, The New York Times reported. Ross said that Dear could have flashes of anger, but usually he would later apologize.
Dear was politically conservative, religious, owned guns and believed that abortion was wrong — but he was not obsessed with any of these issues, Ross said.
“It never, ever, ever, ever crossed my mind” that he would be capable of a mass shooting, Ross told The Times. “My heart just fell to my stomach.”
After their amicable divorce in 2000, Dear lived in a series of trailer homes and remote cabins, mostly keeping to himself.
For a time he lived in a tiny shack outside the small town of Black Mountain, North Carolina.
“He was the kind of person you had to watch out for,” a neighbor told The Washington Post. “He was a very weird individual. It’s hard to explain, but he had a weird look in his eye most of the time.”
Dear also lived in South Carolina, where police released reports of at least seven times that he argued or fought with neighbors, including a “Peeping Tom” charge in 2002 that was dismissed.
In 2014 Dear moved west and bought a small plot of land in the tiny mountain town of Hartsel, just west of Colorado Springs, real estate agent Jim Anderson told The Post.
Police cordoned off the area and carefully searched the broken down mobile home that he lived in without water or electricity or even a sewer, local media said.
Anderson, who brokered the plot sale, said he was shocked when he saw his former client identified as the shooter.
“He seemed okay to me,” Anderson told The Post. “Just like anybody who wanted to buy a piece of land in this one-horse town.”