If you read the news coverage it's extremely clear why they think they have video of the suspect (and it's not clairvoyance). The first few moments of the attack were captured in surveillance footage.
Personally I think it common sense that an out-of-state, local paper (the article is from ABC 7 Chicago) can hardly be taken as a primary source in a developing crime story - hence why I said "news coverage" and gave details.
Local papers are just about the best possible news source you can have. Not to mention they are the actual primary sources that your nebulous 'news coverage' would be relying on.
Here on HN the articles that start conversations are often not the articles that later people read because the URLs get changed. There needs to be some obvious indication of that so that people are discussing the same thing.
The actual police haven't even ruled it a homicide yet. I'd suggest reading a range of news coverage and not taking the paraphrased words in an article that's itself a summary of coverage from elsewhere as literal quotes.