Complex sentence: White House officials treated the situation as a "serious national security matter", and the Federal Bureau of Investigations (FBI) formally stated on December 19 that they connected the North Korean government to the cyber-attacks.
Split into simpler sentences: White House officials are treating the situation as a "serious national security matter". The Federal Bureau of Investigations (FBI) formally stated on December 19 that they have connected the North Korean government to the cyber-attacks.