Where's mommy?

He woke up by a crying baby outside of his apartment room. When he heard someone knocked on his door he finally stood up. When he went outside of his apartment house he saw a teenage girl and a baby. "Is this baby yours?" She ask. Luke shook his head.He dosent even know about the baby. But then he saw a letter. He read it and he cried.
What's in the letter?
Why did he cry?
Whose baby outside the apartment room?