Murder can be federal crime if it involves:

  * the murder is of a federal judge or a federal law enforcement official

  * the killing is of an immediate family member of a federal law enforcement official

  * the murder is of an elected or appointed federal official

  * the killing is committed during a bank robbery

  * the killing takes place aboard a ship at sea

  * the murder was designed to influence a court case

  * the killing takes place on federal property

And in some locations, the federal government holds exclusive jurisdiction, so they better enforce these laws :)