NEW YORK -- Planned Parenthood deals daily with some of America's most contentious issues, and is well accustomed to receiving verbal threats as a leading defender of abortion rights and comprehensive sex education.

Some of the organization's supporters say Friday's deadly shooting at its clinic in Colorado Springs shows that the vitriolic rhetoric could be inspiring actual violence.

"It is time to stop the demonizing and witch hunts against Planned Parenthood, its staff and patients," said Sen. Barbara Boxer.

But critics show no signs of backing away from a multi-pronged offensive against Planned Parenthood, keeping protests and a congressional investigation on their agenda for the coming year.

The man arrested in the attack that killed a police officer and two civilians uttered the phrase "no more baby parts," a law enforcement official said.

Authorities have not elaborated on the gunman's possible motives, but Planned Parenthood said witnesses described him as an abortion opponent. The "body parts" phrase echoed rhetoric that surfaced last summer, when anti-abortion activists began releasing undercover videos they said showed Planned Parenthood personnel negotiating the sale of fetal organs.

The anti-abortion group that made the videos, the Center for Medical Progress, condemned the "barbaric killing spree in Colorado Springs by a violent madman."

Planned Parenthood said any payments were legally permitted reimbursements for the costs of donating organs to researchers, and has since stopped accepting even that money. Though the videos have inspired multiple investigations in Congress and in several states, none has confirmed any law breaking by Planned Parenthood.

Since the videos surfaced, threats have become even more frequent, abortion-rights leaders say.

"We've seen an alarming increase in hateful rhetoric and smear campaigns against abortion providers and patients over the last few months," said Vicki Cowart, president of Planned Parenthood of the Rocky Mountains. "That environment breeds acts of violence."

There have been eight murders and more than 220 bombings and arson attacks at abortion facilities in the U.S. since 1977, according to the National Abortion Foundation. Two Planned Parenthood receptionists were killed in 1994 at clinics in Massachusetts.

Planned Parenthood has remained in the news since the videos were made public, with most Democratic politicians supporting the organization and many Republican leaders assailing it. Republicans have sought to cut off federal funding for Planned Parenthood, and several states have tried to block Medicaid funding to the organization.

All the Republican presidential candidates say they favour restricting abortion rights. Some were asked about the Colorado Springs shootings on Sunday's talk shows.

Mike Huckabee condemned the attack as a "despicable act of murder" and said "what he did is domestic terrorism," but then equated the killings to the abortions Planned Parenthood provides.

"There's no excuse for killing other people, whether it's happening inside the Planned Parent headquarters, inside their clinics, where many millions of babies die, or whether it's people attacking Planned Parenthood," Huckabee said on CNN's "State of the Union."

Such comments are "unconscionable," said Dawn Laguens, executive vice-president of Planned Parenthood.

"It's not enough to denounce the tragedy without also denouncing the poisonous rhetoric that fueled it," Laguens said. "Instead, some politicians are continuing to stoke it."