To begin with, the investigation did get considerable information from the Associated Press indirectly through its contacts and its e-mails that it reported to the Canadian officials who initially heard from the Associated Press. It was very clear that the Associated Press, as is standard practice among most journalists, never discloses its sources, so in a sense going to the Associated Press and asking them who sent it is a bit redundant. It's just not going to happen.
A second and more important aspect is that when the Canadian officials were inquiring as to the validity of the document and whether it was actually a real government document, they were told that the Associated Press had received a facsimile copy and had retyped the copy they received. Now they didn't say it was to protect their source, but it's self-evident that's exactly why they did it, so picking up the telephone and asking the Associated Press who sent the fax when they had already clearly indicated they had taken the time to retype it was a non-starter.
In effect, the Associated Press, at least to two different individuals, did indicate the source of the document to the extent they were going to.