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Terror has hit a new low.

The Manchester terror attack -- in which a suicide bomber killed 22 and injured more than 100 at an Ariana Grande concert -- has been among the most ruthless attacks due to the victims' ages. Up until Monday's bombing, radical Islamic terrorists have avoided targeting children. But ISIS, which claimed responsibility for the suicide bombing, has seemingly decided to break the long-standing moratorium on directly targeting innocent children.

“I don’t think anything is off the table anymore,” Fred Burton, former special agent and vice president of intelligence at geopolitical research firm Stratfor told Fox News. “We have reached a new low.”

Other terror experts say that ISIS and other radical Islamic groups have a frequently expressed rationale for targeting Western youth.

“Both ISIS and Al Qaeda propaganda have long and often referred to the suffering and depredations inflicted by the West on Muslim women and children in particular,” Bruce Hoffman, director of the Center for Security Studies at Georgetown University, told Fox News. “So the attack on a venue and an event especially heavily attended by young children makes sense in that respect.”

Shortly after the attack ISIS supporters took to the internet to revel in the fact that innocent Western children were killed. On Tuesday an article was circulated on pro-ISIS channels, on the messaging app Telegram, under the hashtags #Manchester and #Britain, according to the Middle East Media Research Institute.

The author, calling herself Umm Omar Al-Iraqiyyah, wrote that the attack was a reprisal for the killing of innocent Muslims and that it was justified under the Quranic principle of reciprocity.

"If you kill our women and children – we will kill your women and children,” reads a line from Al-Iraqiyyah’s posting.

“This issue is clear as the noonday sun,” the post also says.  “A Muslim is allowed to inflict punishment equivalent to the harm that was done to him. Therefore it is permissible to kill the [enemy's] children, women and elderly. This is justice. We kill their women, children and elderly as they killed our women, children and elderly, in order to break their hearts and humiliate them, in light of the verse, 'So whoever has assaulted you, then assault him in the same way that he has assaulted you [Koran 2:194]'”

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This image shows people running through Manchester Victoria station after an explosion at Manchester Arena. in Manchester England, Monday May 22, 2017. An apparent suicide bomber set off an improvised explosive device that killed over a dozen people at the end of an Ariana Grande concert on Monday, Manchester police said Tuesday May 23, 2017. The station is very near the arena. (Zach Bruce/PA via AP)

The author also cited a hadith, or tradition that quotes Muhammad, that tells the story of how he punished two men for killing a shepherd by executing them.

"If this is the obligatory form of reprisal among Muslims, and this is how [the principles of] reciprocity and equality before the law are realized, then the infidels should be the first ones who deserve [such treatment]. We have the right to punish them in the same way they harmed us, and do to them what they did to us,” Al-Iraqiyyah also says in the post. "Shari'a law affirms the principle of reciprocity... The [abovementioned] verses are general and refer to all situations... But when they violate the honor of our women, should we violate the honor of their women?”

She concludes with a line from a well-known Arabic poem.

"If the scorpion returns, we shall return to it – and the shoe is ready [to stomp it].”

Manchester, located 160 miles northwest of London, is one of Britain's largest cities. The attack was the deadliest in Britain since four suicide bombers killed 52 London commuters on subway trains and a bus in 2005.

The claim of responsibility by ISIS echoed others the group has made for attacks in the West, but its vague details left open the possibility it was just an opportunistic attempt at propaganda.

Stratfor’s Burton says this terror attack has struck a particular nerve across the globe.

“When an attack like this occurs, it can touch the heart of a nation,” he said to Fox News.

He also points out that the target was likely planned.

“I do believe it was intentional to attack children,” Burton said. "It’s been my past experience as an agent that several values are looked at by terrorists planning an attack. The location, the victims involved, the event, with an American singer, were all likely considered when they were looking for their target.”

Hoffman said the Manchester bomber was more likely concerned with the location when plotting the attack and that it may have been timed with President Trump’s first overseas trip as well as other upcoming events.

“I think the bomber was looking for a suitably crowded, public venue to carry out an attack both while President Trump was visiting Saudi Arabia, Israel and the Vatican -- longstanding targets of ISIS threats and opprobrium -- and in the runup to the British national elections in two weeks, and the Grande concert fit the bill,” he told Fox News.

“Manchester has always had a radical community. It was the target of a major 1999 Al Qaeda bombing plot, the infamous Al Qaeda ‘Manchester Manual’ was discovered there, and a 2010 plot to attack that city was derailed."