Alan Henning family: 'Numb with grief' following ISIS beheading

News of Henning's beheading was made public Friday in a video released by ISIS, which blamed the killing on the UK for joining the U.S.-lead bombing campaign against the group in Iraq and Syria. In the same video, the group threatened the life of American aid worker Peter Kassig.

UK Prime Minister David Cameron called Henning's murder "absolutely abhorrent" and "completely unforgiveable," saying Britain must work with its allies to defeat ISIS.

"What we see with this organization is there's no level of depravity to which they will not sink. No appeals made any difference," he said, speaking Saturday at the Prime Minister's country house retreat, Chequers.

"...The fact this was a kind, gentle, compassionate and caring man who had simply gone to help others, the fact they murdered him in the way they did, shows what we are dealing with, and this is going to be our struggle there. With others, we must do everything we can to defeat this organization"

But Cameron also warned the fight against ISIS, also known as ISIL, would not be quick.

The Prime Minister said he had held briefings with senior military and intelligence advisers and others, "to make sure we do everything that we can, whether that is taking down these hateful videos from the Internet, whether it's stopping preachers of hate, whether it's prosecuting those who glorify terrorism, whether it's working with others to defeat ISIL in the region."

The news of Henning's beheading came just days after Britain joined the U.S.-led coalition against ISIS, following overwhelming approval from Parliament.

UK jets began flying reconnaissance flights over Iraq a week ago, and on Tuesday dropped their first missiles on an ISIS heavy-weapon position and an armed pickup truck in Iraq, according to the UK Defense Ministry.

The White House released a statement condemning Henning's murder and vowing to work alongside the United Kingdom and its allies to "degrade and ultimately destroy ISIL."

Henning appears to be the fourth Westerner to be beheaded on camera by ISIS.

This summer, ISIS beheaded American freelance journalists James Foley and Steven Sotloff -- showing their gruesome killings in videos posted online. ISIS then claimed its first British victim, aid worker David Haines, according to video that appeared online on September 13.

After receiving an audio file with her husband's voice, Henning's wife attempted to contact those holding him. When she was unable to reach those who sent the file, she made public pleas last month to ISIS to spare her husband's life -- an appeal joined by voices of Muslim leaders around the world.

But those pleas were ignored.

"On behalf of the entire family, I want to thank everyone who campaigned for Alan's release, who held vigils to pray for his safe return, and who condemned those who took him," she said, according to the statement.

"Your efforts were a great support to us, and we take comfort in knowing how many people stood beside us in hoping for the best."

A taxi driver from near Manchester in northern England, the 47-year-old Henning was part of a team of volunteers who traveled to Syria in December 2013 to deliver aid -- food, water and medical supplies -- to people affected by the country's devastating civil war.

He was abducted by masked gunmen the day after Christmas, other people in the aid convoy said.

"Alan was a decent, caring human being. His interest was in the welfare of others," Barbara Henning said in the statement. "He will be remembered for this and we as a family are extremely proud of him and what he achieved and the people he helped."

At a ceremony at Manchester Central Mosque on Saturday, a joint statement mourning the loss of Henning was read out on behalf of mosques throughout the north of England.

"Alan Henning was our local and national hero," it said. "We will remember him as a tireless and selfless volunteer aid worker whose only concern was to help people in need.

"...The killing of Alan Henning was a cowardly and criminal act of appalling brutality by a group who do not represent Islam at all and in fact are an insult to the Islamic faith."

Some 200 mosques and organizations put their names to the joint statement, together representing about a million people, according to mosque officials.

Talha Ahmad, a spokesman for the Muslim Council of Britain, told CNN on Saturday that the organization was "devastated and disgusted" by Henning's murder, which he called "the latest round of despicable violence by ISIS."

The murder, according to Ahmad, confirmed that ISIS is "not a religiously inspired organization" but rather a militia group "hell-bent on inflicting pain and cruelty."

He does not believe a very large number of people sympathize or are even prepared to tolerate the kind of attitude shown by ISIS, he said.

"The challenge we have is to make the case again and again that however tempting, however lucrative it may appear to support a group which stands up to the Western 'bully,' if you like, the reality is ISIS do not represent a hope," he said.

One heartening thing to come out of the tragedy of Henning's death, he said, is that Muslims "of all shades and colors and theology, have come together to condemn ISIS and we just need to keep doing that."

UK authorities believe at least 500 British citizens have gone to Iraq and Syria, many of them to fight with ISIS and other Islamist groups -- and that most will try to return, bringing their extremist views with them.

Cameron last month laid out new measures to tackle the threat posed by would-be UK jihadists, days after Britain raised its terror threat level from "substantial" to "severe."