The CIA needs the military to do something like this. If it were done from a surface ship, even a smaller diving vessel, there'd be a record to show it was in the area and could have done it. The only covert means is by submarine where you release divers to do the operation. That is limited to just a few nations currently.
Sweden, by the way, has a string of hydrophone listening stations along their coast because of old Soviet intrusions by submarines. So, they'd likely have the data on a submarine being in the area and doing this unless that submarine were very careful and knew beforehand how to avoid detection.
As the pipeline is only at about 100 feet down where it was damaged, it'd be pretty easy to dive the site. But you'd need either an ROV or good sonar to know for sure you were on the pipeline before diving it. You couldn't send divers down randomly to do this sort of mission. That again argues for a very sophisticated operation by a top notch military.
