Since the 1973 Supreme Court Decision which legalised abortion in the US, the campaign against abortion has escalated from picketing of clinics to incidents where extremists have carried out arsons, bombings and murder.

According to the US-based National Abortion Federation (NAF), the first abortion provider was murdered in 1993. Dr David Gunn, 47, was shot three times in the back and killed as he got out his car while a demonstration took place outside his clinic. The gunman, Michael F Griffin, was sentenced to life in prison for the murder.

Since then, there have been 11 murders and 26 attempted murders.

The most recent incident happened last November, when a gunman armed with a rifle stormed a clinic run by Planned Parenthood, which provides reproductive healthcare including abortion, in Colorado Springs and opened fire. Two civilians and one police officer died.

Robert Dear, who has admitted to the murders, said in an interview last month: "I picked Planned Parenthood because it’s murdering little babies."

According to the NAF, other tactics used by extremists have included vandalising clinics and sending anthrax threat letters to clinic staff.