12/21: Gunman said to seek vengeance with NYPD cop killings; WWI enemies turned battlefield into Christmas celebration

The NYPD is tracing the steps of a gunman who killed two of its officers back to Baltimore, where he shot his ex-girlfriend earlier in the day. Police say the killer may have acted out of vengeance over recent deaths of black men during encounters with police; and, for a century, people have told the tale of a yuletide ceasefire soccer game in the middle of World War I. German and British soldiers briefly stopped shooting at each other, met and mingled in the no-man's land between their lines.