If you identify someone as a woman and add that she is trans, that should always mean that she was AMAB (assigned male at birth) and now presents as female.

One should always use "the gender the person presents as" when referring to that person, as anything else is deeply disrespectful (to put it mildly). Adding the detail that the person is transgendered provides information about their past.

That said, it seems like in this case the news was wrong. The shooter was apparently "FtM" -- AFAB, transitioned to male at some later point. But the news identified the person as a woman, which from what people are saying in this thread, is misgendering. Some are saying that it could be a accident -- it's possible that official records, like their driver's license, were still listing the person as female.