http://www.dawn.com/wps/wcm/connect/Dawn%20Content%20Library/dawn/news/world/us-calls-it-attack-on-sri+lanka-pakistan-relations--bi
“Others, however, noted that if the aim was to hurt relations between Pakistan and Sri Lanka, which is a big purchaser of arms and ammunition from Pakistan, then there could be two obvious suspects: militant groups backed by Indian intelligence agencies or Sri Lanka’s Tamil separatists.”
“The other possibility – the involvement of Tamil separatists – could not be ruled out either, particularly after the Sri Lankan government’s recent victories against the LTTE, which appear to have decimated the terrorist group.”
And this article is written in a way to promote a break between two countries. As said earlier a good guess may be, this attack is originated by LTTE and carry out by another group of terrorist? Anyway let us see what Pakistani law enforcement will find. More may come in other part of the world who opposes LTTE for a separate state in Ceylon.