Final answer:
The correct word to fill in the blank is 'accomplice' because it means a person who helps someone commit a crime or wrongdoing.
Step-by-step explanation:
The correct word to fill in the blank is accomplice because it means a person who helps someone commit a crime or wrongdoing. In the given context, the thief was caught, but his accomplice escaped, indicating that the thief had someone aiding them in their actions. For example, in the sentence provided, the officer talks about catching the thief but being concerned about the escape of the thief's accomplice. This suggests that there was someone assisting the thief during the crime.