Final answer:
The police suspected the thief worked at the museum because the alarm and security cameras were off when the theft took place and they talked to all the people who worked at the museum.
Step-by-step explanation:
The detail that explains why police suspected the thief worked at the museum is that the alarm and security cameras were off when the theft took place. This suggests that someone familiar with the museum's security system may have been involved. Additionally, the detectives talking to all of the people who worked at the museum would help gather information and potentially narrow down any suspicious behavior or connections to the crime.
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