By Nick Triggle
Health reporter, BBC News
The killing of an abortion doctor has once again highlighted the aggressive and emotional nature of the abortion debate in the US. Why does the issue elicit such a strong response compared to countries such as the UK?
There are about 800,000 abortions a year in the US
It is perhaps a mark of the fervour that characterises the abortion debate in the US, that even President Barack Obama is not immune.
One of the few times he has faced an angry audience since taking power was during an appearance at Indiana's Notre Dame, one of the largest Catholic universities in the country.
He was heckled by anti-abortion activists over his decision earlier this year to lift restrictions on funding for abortion.
Indeed, heckling and protests over abortion are much more commonplace in the US than they are in the UK.
With hundreds of religious radio stations across the country, the anti-abortion movement enjoys a much higher profile than its equivalent on the other side of the Atlantic.
But as well as getting more attention, there is also more violence associated with the issue - as the shooting of Dr George Tiller demonstrates.
While many anti-abortion organisations have come out and condemned the killing on Sunday, the fact remains that violence and intimidation have remained a constant thread in the history of the modern-day movement.
According to data gathered by the National Abortion Federation, a pro-choice group, there had been at least eight killings in anti-abortion protests, 17 attempted murders and 400 death threats.
Bombings and arson attempts of abortion facilities also happen on a regular basis with cases often reaching double figures every year.
The most famous was probably the Christmas Day bombings which targeted three clinics in 1984 with the perpetrators dubbing them a "birthday gift for Jesus".
The abortion debate in the US has been gathering pace ever since a Supreme Court judgement in 1973.
The landmark Roe v Wade ruling decreed that most existing state abortion laws were unconstitutional and the termination of pregnancy should be made legal.
Since then there has been a gradual rowing back from that position - although laws do vary from state to state.
Restrictions are now in place on everything from funding and insurance cover to late-stage abortion known as partial birth by its opponents as the foetus is forced out of the womb.
This is relatively rare with only a handful of clinics, including Dr Tiller's, willing to carry out such procedures.
In total, only about 2,000 of the 800,000 abortions carried out in the US each year are classed as partial birth.
One of the obvious differences between anti-abortion movements in the US and the UK is the level of involvement of religious groups.
With half the US population regular church-goers, everything from sexual abstinence and euthanasia campaigns to the abortion debate has been dominated by religious groups.
Dr Joe Cassidy, an expert in medical ethics at Durham University, believes this has played a role in the violence that has been seen.
"The debate in the US has been couched in the terms of defending the defenceless. That gives some people a feeling that violence is therefore justified.
"That sort of debate has never taken off in the UK. There is a feeling that it should be left to the government.
"But in the US there is a fundamentalist religious fringe which believe it is their duty to take matters into their own hands."
Dr Cassidy also said there was a marked difference in what opponents were trying to achieve. "In the US there is a will to ban abortion that is not present in the UK.
"Here the campaigners have been focusing on introducing more restrictions, which would be seen as selling out in the US."
But campaigners in the UK do not believe these differences really account for the violence that has attached itself to the US movement.
John Smeaton, national director of the Society for the Protection of Unborn Children, who has condemned the killing of Dr Tiller, said: "I think it is more to do with the levels of gun crime in the US.
"Guns are more readily available so by the law of averages people providing abortions are going to be caught up.
"What is important to remember is that these acts of violence are not carried out by organisations but by lone individuals.
"I know many of the leaders of these organisations in the US and I know they share the same values as we do in the UK.
"Killings go against everything we stand for - the right to life."