Photo of the mosque after the attack.

Another suicide bomb hit Saudi Arabia on Thursday, this time at a mosque inside a Special Forces headquarters in Asir, southwest of Saudi Arabia, killing 15 people and injuring nine. The attack, by ISIS, was carried out by Abu Sinan Al Najdi.

This is the fourth bomb attack on a mosque in Saudi Arabia, but the first on a non-Shia mosque. Last year, in November, eight people were killed in the Eastern province of Al Ahsa, when gunmen attacked a Shia community centre. The Al-Qaeda affiliate, ISIS, claimed responsibility for two following attacks in the Eastern Province, one in Qatif killing 21 worshippers, and the second in Dammam, which killed four people. Another bombing on a Shia mosque in Kuwait in June killed 27 people, was also claimed by ISIS. On July 16, a man was killed and two security officers were injured in a suicide car bombing in Riyadh.

“While a group of the personnel of special emergency forces in Asir region were performing Al-Dhuhur (Noon) prayer together at the forces headquarters,” reported the Saudi Press Agency, quoting an Interior Ministry spokesman.

Check out our coverage of the attack on Global Voices Checkdesk

The attack was at the headquarters of the special emergency forces in Asir, close to the Yemeni border. The ISIS said in a statement the bomber had used an explosives belt. The attack was claimed in online statements by a new IS-affiliated group, Al-Hijaz Province.

Saudi security forces launched a countrywide search that resulted in the arrest of more than 400 people alleged to be linked to ISIS, following last month's attack, the Interior Ministry said on July 18.

On social media, news of the Asir attack made headlines.

Alhadath KSA was the first account to report the attack saying:

#عاجل تفجير انتحاري بمبنى قوات الطواري بـ #عسير واستشهاد عدد من رجال الامن وهناك مصابين . #السعودية

— السعودية الحدث (@ALHADATH_KSA) August 6, 2015

A suicide bomber blows himself up in the emergency forces building in Asir. A number of security forces martyred and others injured

Footage inside the mosque shows Saudi security forces inspecting the site of the explosion:

Saad Alshahrani shares a photograph showing the Prince of Asir Faisal bin Khalid visiting the mosque after the attack:

الامير ” فيصل بن خالد يحضر الان لمقر الحدث فـ طوارئ عسير بعد الحادثة المؤلمة .. #تفجير_طواري_عسير pic.twitter.com/M4QqdwYV9t

— سعد الشهراني (@Saad_t8) August 6, 2015

Prince Faisal bin Khalid is during at emergency visit to the mosque after the painful attack.

People have shown respect and unity with the victims on Twitter. Akl Albahli writes:

حينما ترى عنف التشدد في قضايا إجتماعية تافهه تدرك مدى خطورته حينما يتبنى مشروع تهديم وطن عبر تحريكه كأدوات عمياء #تفجير_ارهابي_في_عسير

— عقل الباهلي (@AklAlbahli) August 6, 2015

When you see violent extremism in silly social problems you realise the extent of the danger when they adopt a program to wipe out the nation by using extremists like blind tools

Faeq Muneef reacts to the killing of people while praying:

تخصص الإرهابيون في تفجير المصلين الساجدين، حتى لكأن لهم دين شيطاني يأمرهم بتخويف المسلمين، ومنع الناس من عبادة الله #تفجير_طواري_عسير

— فائق منيف (@Fmuneef) August 6, 2015

Those terrorists are becoming experts in blowing up people who are praying as if they are moved by a Satanic religion which orders them to terrorise Muslims and prohibit people from their religious duties

Saudi writer Abdulla Alami adds:

تغييرالعقلية الذهنية الارهابية المتراكمة عبر 30 عام يتطلب أكثر من تنظيف وتطوير المناهج، ومراقبة المنابر، وملاحقة المخالفين #تفجير_طواري_عسير

— عبدالله العلَمي (@AbdullaAlami) August 6, 2015

Changing the mentality of terrorists that has accumulated over 30 years requires more than developing the [school] curriculum, and monitoring the pulpits of mosques, and the prosecution of offenders.

Minister of Labor Mufarrij Al-Haqbani assured:

نقف صفاً واحداً كالبنيان ضد من يحاول أن يسيء لهذا الوطن الشامخ بدينه وقيادته ورجال أمنه وشعبه #تفجير_طواري_عسير

— د. مفرج الحقباني (@MofarrigHaqbani) August 6, 2015

We stand united against anyone who offends this country and its men, leaders, security forces and people

Saudi Shiite cleric Sayyed Hassan Al Nemer added:

الإرهاب الأسود يسفك الدماء بغير حق من #الدالوة إلى #القديح مرورا ب #الدمام و #القطيف وأخيرا #تفجير_طوارئ_عسير حفط الله الوطن ورحم الشهداء.

— السيد حسن النمر (@alnemernet) August 6, 2015

Black terrorism is shedding blood unjustly from Dalalwa [the Ahsa neighbourhood where the first attack took place] to Qudaih [in Qatif] passing through Dammam and Qatif, and lastly the blast in security forces quarters in Asir. May God bless this nation and its martyrs.

Saudi journalist Adhwan Alahmari asks:

#تفجير_طوارئ_عسير سبقها استهداف مبنى الأمن العام قبل 11 عاماً. هل تم تنفيذ حكم الإعدام في المتورطين؟ أم تمت مناصحتهم فحسب؟

— عضوان الأحمري (@Adhwan) August 6, 2015

Before the mosque attack, general directorate of general security was targeted 11 years ago in Riyadh. Were those involved handed the death penalty? Or they were advised [through Saudi Arabia's rehabilitation program]?

Al Marid says the whole of Saudi Arabia is now being targeted by the ISIS and not just one sect:

#تفجير_طواري_عسير اليس هناك من شك بعدهذه الجريمةان المستهدف ليست فئةمن فئات المجتمع بعينها بل ان المستهدف هو الوطن بكل مكوناته وطوائفه/إصحوا

— المارد (@ALMARID_99) August 6, 2015

There is no doubt after this attack, it is not only a faction of society that is targeted, but it is the country as a whole with all its sects. Wake up!

Saudi writer Nabeel Almojil notes:

#تفجير_طوارىء_عسير قلناها مليون مرة جففوا منابع الاٍرهاب والتكفير والأمر ليس بحاجة لعمليات استخباراتية رموز ومنابع الإرهاب بيننا!!

— نبيل_المعجل (@NabeelAlmojil) August 6, 2015

We have said it a million times. Dry up the sources of terrorism. This doesn't need spy operations as their symbols and sources are amongst us

And Saudi lawyer Khalid Abo Rashed concludes:

بالأمس شيعة واليوم سنة لم يسلم المدني ولا رجل الأمن فالمستهدف واحد وهو الوطن وهذا مختصر الكلام #تفجير_طواري_عسير

— خالد أبو راشد (@KhalidAboRashed) August 6, 2015

Yesterday, the Shias were bombed, today the Sunnis. No one is immune, whether a civilian or a policeman. Eventually, the only target is the nation.