Libel is written (known) lies that harm or cause defamation. Slander is spoken or recorded. Blaming Trump, blaming NRA, Blaming Nikolas Cruz, etc.. if any are proven false, would fall under one of the above. If CNN knowingly passes on lies the CIA feeds or Sheriffs office passed them, it would still apply.
The problem with the current libel laws now is you have to prove they knew it was a lie (which can be covered with a lie) AND you have to prove harm. All those sponsors dropping for the NRA would hit that mark, but its harder to prove harm done to Trump.