'Sonic attacks' suffered by US diplomats caused by microwave energy, study says

Taking the word "likely" out of the title changes the overall meaning and intensity of the title and article. There is no smoking gun implied. A path to the smoking gun, perhaps.

Even in the article, there is plenty of doubt that it was microwave energy. What they do say is that the symptoms suffered by the people in question appear to match symptoms that can be attributed to microwave radiation.

They do point out that a couple of state individuals known for working on exactly this type of weapon were in the city at the same time. That could be a coincidence. It's surely a thing that makes you go 'hmm...'

That is not at all suggesting that it was factually caused by microwave energy. It's a working theory. They actually do not know. But they do find the symptoms similar enough to look in that direction.

The word "likely" is a very important part of the original title, and the article in general.