I found 3 potential problems, so there may be a typo in one of the sentences you posted. Number 4 may be okay as is, but I've included it here, and you can compare to your assignment.
3 This morning, the police chief had given me an important assignment.
Corrected: This morning, the police chief gave me an important assignment.
4 The police called me to help because i had cracked many difficult cases in the past.
Corrected: The police called me to help because i have cracked many difficult cases in the past.
5 It was a difficult assignment, and investigators were stumped. so they ask me to help.
Corrected: It was a difficult assignment, and investigators were stumped, so they asked me to help.