The phrase "actually the source of" is correct and usable in written English.
It can be used to clarify or emphasize the origin or cause of something in a discussion or explanation.
Example: "After further investigation, we found that the issue was actually the source of the problem we encountered last week."
Alternatives: "truly the origin of" or "genuinely the cause of".
That advertorial – advertising content that looks like editorial – is actually the source of BuzzFeed's financial progress, according to Peretti.
The very program that is supposed to be their safety net is actually the source of the problem, experts say.
Second, Paulson and others have also begun to say that Lehman's failure wasn't actually the source of our current problems, but rather a sign of them.
(Thomas Cushman, one of the editors of "Terror, Iraq, and the Left," said of Hitchens, "What's great about him is that being despised is actually the source of his creativity").
Meanwhile, they figure out that the oddball passenger, a frustrated janitor at a Times Square hotel, is actually the source of the series of ghost appearances, which are part of his dastardly plot for world domination.
The maker of that video, Rifaat Hamdia, 30, said he wanted to show the world that the tunnels many think of as dedicated to Hamas arms smuggling are actually the source of basic goods like cooking oil and detergent.
But the dates, which are much more precise in the US Southwest due to the use of dendrochronology (dating sites by annual growth rings from trees), were too late for the Classic Maya to actually be the source of the cacao that made its way up to the US Southwest.
However, if our hypocenter of the earthquake is correct, the Marakau fault cannot actually be the source of the earthquake, because the Marakau fault would need to have an average surface-to-hypocentral dip of only 40°, far shallower than the 60° dip we determined for the rupture.
Spambots sometimes try random emails until they get a hit, the users may have malware on their PCs which has captured their email through other methods (like a keylogger, for example), and there's a chance that another third-party app which integrates with Dropbox may actually be the source of the problem.
But a tax return doesn't tell us who actually provided the source of income.
Although queue-jumping is a big taboo in England, we have other social rules that come into play, such as not making a scene, not drawing attention to yourself in public, not confronting strangers, and always moaning about a problem rather than actually addressing the source of the problem.
