It was a Pakistani according to Arab media reports. This is not something common in Saudi Arabia, Saudi should do indepth investigation. Like JUBA said, there is only one people who have major interest in causing revolt/civil strife in Saudi Arabia. And they will use Arab Shias to the last one to achieve their agenda.
Nothing is official yet. Regardless of who did it this is extremely rare for KSA (second such attack in KSA's history). Outside of those 2 attacks (the first by a a small ISIS cell last year) the only terrorist attacks in KSA have been done by Al-Qaeda on mostly American soldiers, Western expats and Saudi Arabian officials (policemen mostly). There have never been any sectarian infighting between Saudi Arabian Sunni's (Shafi's, Hanbalis, Malikis, Hanafis), Sufis, Shias (Twelver, Zaydi, Ismailis) etc. The only thing I recall was some Ismaili riot in the Najran province ages ago (1990's).
You never hear of any Sunni-Shia-Sufi sectarian infighting in Hijaz for instance between locals. Or even any expats.
Definetely it would suit the enemies of KSA to try and make the Shia population in the Eastern Province (they are not a majority) hostile against the regime and thus this way create instability in KSA and weaken it. Does not take a genius to figure this out considering the climate of the region.
Anyway KSA's has an excellent intelligence and we all know that the justice system is quite harsh so there won't be mercy for any terrorists who kill innocent citizens or expats. All of those from the first attack have been sentenced to death.
In the end ISIS and Sheikh al-Nimr types have 1 thing in common. They are against the regime and are willing to use violence to remove it.
I don't care about the regime particularly much or any regime in the world for that matter but changes must come gradually. No need to turn into another Iraq.
The Arabian Peninsula should unite into 1 federal entity anyway. Always said that. With autonomy too for all I care to all religious and ethnic groups if they want that. No problem for me.
In fact I am quite surprised that there have not been many more attacks considering all the unstable countries that KSA borders directly, the huge expat communities etc.