"Elimination of witnesses" makes sense for a cable-tapping mission. You can't let the country know that you tapped their cable. It would ruin the whole point.
On the other hand, if you leave a bunch of dead bodies with bullets in them, a reasonably-competent government is going to figure out that something happened there, and if the cable is near there, it's a reasonably likely candidate...
They don’t necessarily know what the mission was, just that the mission left dead bodies. Furthermore, they don’t know which country did it. There are probably a dozen countries that would do a mission like this against them with various objectives.
You also have to consider the possibility that the mission was intentionally designed as a distraction, such that the purpose was to leave evidence of a mission.
On the other hand, if you leave a bunch of dead bodies with bullets in them, a reasonably-competent government is going to figure out that something happened there, and if the cable is near there, it's a reasonably likely candidate...