Deadline at Dawn by Cornell Woolrich (Author)

One chance in 7,000,000 of getting away! Four hours to make good their escape! She had not the courage to go without him. He cannot go until he finds the person who committed the murder which can so “circumstantially” be traced to his door. And that person was one of 7,000,000 in New York City.
Bricky, the small town girl, meets Quinn, a boy from her own Iowa town, a day too late. The long arm of coincidence brings them happily together — two lost “little” people in the vast city. The long arm of retribution reaches out to separate them forever.
When Quinn, at Bricky’s behest, so that as free people they can take the morning bus back to Iowa, goes to return some money he has just stolen, he finds a dead man in the house. They spend the rest of the night finding the murderer. If they don’t the police will find Quinn!
The story of the night’s adventure is fast moving and tense, building constantly to almost unbearable excitement.