The suspected gunman is Robert Lewis Dear, 59, a law enforcement official told CNN. The official did not provide additional information.
Police captured him, but they're still working to pinpoint his motive -- and make sure he didn't leave any explosives inside or outside the Colorado Springs building.
Bomb technicians are investigating what's believed to be the suspect's vehicle in a parking lot, a law enforcement official said.
It was not immediately clear why Planned Parenthood was the target of the shooting.
One Planned Parenthood official said the motive behind the attack was unknown, but pointed to what she called a "poisonous environment that feeds domestic terrorism" as a possible cause.
"We don't yet know the full circumstances and motives behind this criminal action, and we don't yet know if Planned Parenthood was in fact the target of this attack," Vicki Cowart, president and CEO of Planned Parenthood of the Rocky Mountains, said in a written statement. "We share the concerns of many Americans that extremists are creating a poisonous environment that feeds domestic terrorism in this country. We will never back away from providing care in a safe, supportive environment that millions of people rely on and trust."
Cowart told CNN she believes all staff and patients from the clinic are accounted for.
"We're still reaching out to confirm individuals, how they are. I believe no one of our staff was severely injured. I also believe at this time that none of our patients were injured," she told CNN.
Officer was a six-year veteran
The slain officer, 44-year-old Garrett Swasey, worked for the University of Colorado in Colorado Springs and was on campus about 10 miles away when the shooting broke out. He went to the scene "in support of an officer under fire," the school's chancellor said in a statement.
Swasey was a six-year veteran of the university's police force.
Four civilians and five police officers were taken to hospitals with gunshot wounds. The injured victims were in good condition Friday night, police Lt. Catherine Buckley said.
"The officers, they're obviously in some pain, but thankfully they're alive and talking to us, and they're heroes ... The courage they displayed today saved many, many lives -- no doubt," Colorado Springs Fire Chief Christopher Riley said.
In a statement released late Friday, Planned Parenthood CEO Cecile Richards thanked law enforcement officers.
"Our hearts go out to the families and loved ones of the brave law enforcement officers who put themselves in harm's way in Colorado Springs," she said.
Siege lasted for hours
The announcement of the suspect's arrest came nearly six hours after someone first called 911 around 11:30 a.m. (1:30 p.m. ET) and reported a shooter was inside the Planned Parenthood building, which is near a shopping center and numerous offices.
For hours, police reported shootouts with the gunman, warning people to stay inside nearby businesses and seek shelter.
The shooting brought life to a standstill as police closed roads and people were trapped inside businesses for hours in a busy corner of the central Colorado city, which has more than 400,000 residents.
First, Tessa Smart saw a look of concern flash across a patient's face in a nearby chiropractor's office where she works. Then she heard the gunfire.
"I heard a bullet go right by the door and hit something," she told CNN. "And I ran back to my desk and grabbed a bunch of keys and fumbled at the door trying to find one to lock it. ... As I was doing that I was seeing the police commotion outside, police running everywhere, ducking behind cars with guns."
Denise Speller, who works at a salon, said she heard at least 10 gunshots go off and saw a police officer get hit.
"It was terrifying," she said.
Joan Motolinia said he got a phone call Friday afternoon from his sister, who was inside the clinic.
"I heard the shooting," he said, choking up as he described the situation to reporters. "She couldn't say too much because she was afraid."
In a nearby grocery store, an announcement over the intercom told shoppers they weren't allowed to leave. They huddled with staff at the back of the supermarket.
It was shocking to watch SWAT teams while holed up inside a nail salon, witness Jill Lavelle said.
"Seeing the SWAT members squatting down in their shields and their riot gear, with their automatic rifles out, that was very ... I never have experienced anything like that before," she said.
Despite initial fears that the shooter could be on the loose outside, authorities now believe he remained inside the Planned Parenthood building throughout the siege, Buckley said.
Several injured victims made their way into a nearby bank, she said.
Attack comes as Planned Parenthood faces criticism
Hours after the shooting began, video from CNN affiliates showed people in medical scrubs and white coats being escorted from the scene.
Planned Parenthood is a national health care provider that delivers reproductive health care and sex education to women and men throughout the United States. The organization runs nearly 700 health centers throughout the United States, according to its website.
The group says each year 2.7 million people in the United States visit its health centers for health care services and information. Abortion is among the services Planned Parenthood clinics provide, an issue that's made the organization a target in the past.
It's faced sharp criticism in Washington and from some Republican presidential candidates on the campaign trail after an anti-abortion group released a series of videos alleging that Planned Parenthood engaged in the selling of fetal organs and parts for profit.
Planned Parenthood has strongly disputed the videos and contended the tapes provide a distorted account because they are heavily edited.
At least three Planned Parenthood buildings have been vandalized since September.
Last year, abortion providers in the United States saw 12 incidents of vandalism, one burglary, one invasion, one arson attack, one death threat and four stalking incidents, according to the National Abortion Federation