Planned Parenthood said that news reports that the gunman who attacked its Colorado health clinic had uttered "no more baby parts" during his arrest showed the suspect was motivated by an anti-abortion agenda, on Sunday.

The remark attributed to suspect Robert Lewis Dear was an apparent reference to Planned Parenthood's abortion activities and its role in delivering foetal tissue to medical researchers, a hot-button issue in the 2016 race for the presidency.

"We now know the man responsible for the tragic shooting at PP's health centre in Colorado was motivated by opposition to safe and legal abortion," the organization said on Twitter.

Conservatives have accused Planned Parenthood, a non-profit that provides a range of health services, including abortion, of illegally selling baby parts, an accusation it has strenuously denied.

Dear, a 57-year-old South Carolina native who moved to Colorado, made the remarks during his arrest after a standoff lasting several hours at the Colorado Springs clinic on Friday, NBC News and other media outlets reported, citing unnamed law enforcement authorities. Reuters was unable to independently confirm the reports.

While Dear's reported remarks could hint at a possible motive for the rampage, which killed three people and wounded nine, NBC's sources stressed that investigators were still not sure why the attack was launched.

Authorities have steadfastly declined to discuss a motive, saying their investigation was still under way.

Colorado Springs police, in a tweet on Sunday, said unofficial leaks could jeopardize the investigation and prosecution, without specifically mentioning the words attributed to Dear.

Dear, who appeared to have moved to a remote community in Colorado last year, has been jailed ahead of a court appearance scheduled for Monday.

The shooting is believed to be the first deadly attack at an abortion provider in the United States in six years. The Colorado Springs centre has been repeatedly targeted for protests by anti-abortion activists.

At least eight workers at clinics providing abortions have been killed since 1977, according to the National Abortion Federation. The most recent was in 2009 when physician George Tiller was shot to death at a church in Wichita, Kansas.

While calling the shooting "an incredible tragedy," Republican presidential candidate Mike Huckabee on Sunday dismissed talk that harsh anti-abortion rhetoric may have contributed to the attack.

"What he did is domestic terrorism," the former Arkansas governor told CNN, referring to the gunman.

"There's no excuse for killing other people, whether it's inside ... Planned Parenthood clinics, where many millions of babies die, or whether it's people attacking Planned Parenthood," Huckabee said.

Carly Fiorina, the former Hewlett-Packard chief executive who is also seeking the Republican nomination, said on Fox News it was "typical left-wing tactics" to demonize opponents of abortion or the "sale of body parts" because of what she said was "obviously a tragedy."

Planned Parenthood responded in a statement.

"It's not enough to denounce the tragedy without also denouncing the poisonous rhetoric that fuelled it," said Dawn Laguens, Executive Vice President of Planned Parenthood Federation of America. "Instead, some politicians are continuing to stoke it, which is unconscionable."

Planned Parenthood came under fierce criticism this year after some of its officials were secretly recorded by an anti-abortion group discussing compensation for providing human tissue from aborted foetuses to researchers.

Critics say the footage is evidence that Planned Parenthood has illegally sold baby parts. The organization denies the accusation, saying that some affiliates have donated tissue for research and were paid a small fee to cover costs.

Planned Parenthood recently announced it was discontinuing the practice, aiming to tamp down the controversy, but critics say is an admission of guilt.

The Centre for Medical Progress, which produced the videos, issued a short statement on its website on Sunday, saying it "condemns the barbaric killing spree in Colorado Springs by a violent madman."

The attack led Governor John Hickenlooper to call for both sides of the debate over Planned Parenthood's activities to "tone down the rhetoric."

"I think we should have a discussion at least urging caution when we discuss some of these issues, so we don't get people to a point of going out and committing violence," the Democratic governor told CNN, describing the rampage as "a form of terrorism."

The national security and civil rights divisions of the US Justice Department have joined state and local authorities in investigating the shooting, Justice said in a statement. That raises the possibility that the federal government may bring terrorism or civil rights charge, or both.

TWO CIVILIAN CASUALTIES

Colorado Springs police on Sunday identified the two civilians killed in the rampage as Jennifer Markovsky, 35, and Ke'Arre Marcell Stewart, 29.

Markovsky was a stay-at-home mother of two young children. Originally from Hawaii, she was at the clinic to support a friend, according to her sister-in-law Julia Miller.

Stewart, a Texas native, was a US Army veteran who served in the Iraq war.

Garrett Swasey, 44, the police officer killed in the attack, worked for the University of Colorado at Colorado Springs. He was a father of two and served as an elder at a local church.

Except for his name and age, police have only said that Dear recently resided in rural Hartsel, about 60 miles (96 km) west of Colorado Springs. Official records show that he has a history of brushes with the law, mostly in South Carolina, but no criminal convictions.