BURAIDA (Saudi Arabia) • The men were not hardened militants. One was a pharmacist, another a heating and cooling technician. One was a high school student.

They were six cousins, all living in Saudi Arabia, all with the same secret. They had vowed allegiance to the Islamic State in Iraq and Syria (ISIS) - and they planned to kill another cousin, a sergeant in the kingdom's counter-terrorism force.

And that is what they did. In February, the group abducted Sergeant Bader al-Rashidi, dragged him to the side of a road south of this central Saudi city, and shot and killed him.

With the camera rolling, they condemned the royal family, saying it had forsaken Islam. Then they fled into the desert.

The video spread rapidly across the kingdom, shocking a nation struggling to contain a terrorist movement seen as especially dangerous not just because it promotes violence, but also because it has adopted elements of Saudi Arabia's conservative version of Islam - a Sunni creed known as Wahhabism - and used them to delegitimise the monarchy.

Among the 20 terrorist episodes in Saudi Arabia since late 2014, the killing of Sgt Rashidi was the third in which citizens had secretly joined ISIS and killed relatives in the security services. In each case, they justified their acts by saying Saudi Arabia practised a corrupted version of the faith, a charge aimed at a kingdom that holds itself up as the only true Islamic state.

ISIS, like Al-Qaeda before it, accuses the Saudi monarchy of corrupting the faith in order to preserve its power.

But Al-Qaeda networks in the kingdom were dismantled years ago, and the group's leadership abroad has discouraged killing Muslim civilians. ISIS, however, has been able to infiltrate the kingdom through digital recruiting, and has found devotees willing to kill fellow Sunnis, as well as Shi'ites, to destabilise the monarchy.

In July, a 19-year-old man murdered his uncle, a police colonel, before carrying out a suicide attack near a prison, wounding two guards. In an audio message released by ISIS after his death, he addressed his mother. "Your apostate brother was a loyalist to the tyrants," he said. "Were it not for him, the tyrants would not exist."

Major-General Mansour Turki, a spokesman for the Saudi Interior Ministry, said that terrorist attacks over the past two years have killed scores of people, along with about two dozen militants. In addition, about 3,000 Saudis have joined militant groups abroad, and more than 5,000 have been incarcerated at home on terrorism charges, a large increase in recent years.

Saudi Arabia has a tangled history with Islamist militant groups. For a long time, it backed them as proxy forces to push its agenda in places like Bosnia, Chechnya and Afghanistan. But that largely ended in 2003, when Al-Qaeda turned its focus on the kingdom and staged a series of deadly attacks. Now, ISIS poses a new challenge, by turning aspects of Saudi Arabia's conservative creed against it.

Wahhabism has been moulded over the years to serve the interests of the monarchy, emphasising obedience to the rulers and condemning terrorist attacks, even against those seen as apostates.

Saudi officials reject comparisons between their ideology and that of ISIS, noting that millions of non-Muslims live in the kingdom and that the government is closely allied with the United States and participates in the campaign against the militant group.

They also say that Saudi Islam does not promote the caliphate, as does ISIS, and that senior clerics condemn the terrorist attacks and have branded the group "deviant".

But critics argue that many Saudi clerics have never renounced the aspects of the Wahhabi tradition that ISIS has adopted, especially with regard to Shi'ites, who make up an estimated 10 per cent of the kingdom's 20 million citizens.

Many Saudi clerics consider Shi'ites heretics and accuse them of loyalty to Saudi Arabia's regional rival, Iran. The Islamists have exploited this by repeatedly launching suicide attacks on Shi'ite mosques and then accusing Saudi clerics of hypocrisy for condemning the violence.

As elsewhere in the world, ISIS has relied on social media to reach inside the kingdom, find recruits and dispatch them to attack, often under the noses of their closest relatives. This has made plots hard to prevent, Maj-Gen Turki said, citing the example of a man arrested last year after killing two police officers in a drive-by shooting near Riyadh.

One ISIS supporter had given him the car, and another had provided the gun, but the attacker never learnt their names.

Still, the group has struggled to target the security forces, so it told recruits to kill officers from their own families. Maj-Gen Turki summarised their message as: "You are closer, so no one will know you."

NEW YORK TIMES