In the US it is very hard to win a libel case (as the plaintiff). Basically, you have to prove that the news agencies
intentionally and maliciously libeled you. This is pretty darn near impossible to prove in most cases. If I recall correctly, an example of a case where a libel lawsuit would be successful is one where the new agency makes up something or encourages a source to make up something.
EDITS (replies to below):
Defamation laws in the US are actually very complex and in certain cases ambiguous. Different states have different standards for civil liability. In Gertz v. Robert Welch (https://en.wikipedia.org/wiki/Gertz_v._Robert_Welch,_Inc.), which involved a newspaper publishing lies about a non-public official, the Supreme Court held that state defamation laws had to have the plaintiff prove at the very least fault of some kind on the defendant's part, and that for punitive damages malice must be proved. The court also found that people cannot be made into a public official.
Certain statements are considered to be inherently damaging and fault does not need to be proved.
Regardless, clearly Ryan Lanza does not have a case since law enforcement officials did originally identify him, wrongly, as the suspect. The police were clearly not negligent because the reason they ID'd him as the gunman was because his ID was found on the gunman's body. I don't know if the news actually stated as a fact that he was guilty.