Police officers and fire department personnel lead people who were in a Planned Parenthood center out of an armored vehicle, after reports of an active shooter in Colorado Springs, Colorado, Nov 27, 2015. [Photo/Agencies]

'Domestic terrorism'

As in much of the rest of the country, abortion is a divisive issue in Colorado, figuring prominently in attack ads during last year's US Senate race between incumbent Democrat Mark Udall and Republican challenger Cory Gardner, the winner of the election.

At least eight abortion clinic workers have been killed since 1977, according to the National Abortion Federation - most recently in 2009, when abortion doctor George Tiller was shot to death at church in Wichita, Kansas.

Clinics have reported nearly 7,000 incidents of trespassing, vandalism, arson, death threats, and other forms of violence since then, according to the abortion-rights group.

Hundreds of protesters picketed in front of the Colorado Springs clinic in August as part of a push by abortion opponents to cut off public funding for Planned Parenthood, according to the Colorado Springs Gazette.

Colorado Springs was the scene of a mass shooting on Oct 31 in which a gunman killed three people near downtown before dying in a shootout with police.

The city, home to the US Air Force Academy and the US Olympic training center, is also a hub for conservative Christian groups such as Focus on the Family that oppose abortion.

The attack in Colorado sent jitters across the country. The New York City Police Department reported it had redeployed some of its "critical response" vehicles to Planned Parenthood locations throughout the city. However, it said there were no specific threats to those sites at this time.