Fill in the correct modal verb and form of the infinitive.

There was a bank robbery in town this morning and PC Jones was sent to investigate. He's reporting his findings to the Chief of Police. Complete what he says.

"Well sir, it 1) must have been (be) a professional gang because it was a very clever job. They 2)... (know) exactly what they were doing because they didn't leave even one clue behind them. It definitely 3) ... (be) Freddy Fingers and his gang because they are in prison. I thought it 4)...(be) Harry but he was in hospital at the time of the robbery, so it 5) ... (be) him either. I 6) ... (be) sure, but it 7)... (be) Sly Steve's gang, because they are the only suspects who don't have an alibi; they 8) ... (commit) the robbery. 9)... (I/ bring) them in for questioning, sir?"

Well sir, it must have been  a professional gang because it was a very clever job. They had to know exactly what they were doing because they didn't leave even one clue behind them. It definitely couldn't be Freddy Fingers and his gang because they are in prison. I thought it could be Harry but he was in hospital at the time of the robbery, so it couln't be him either. I can't be  sure, but it must be Sly Steve's gang, because they are the only suspects who don't have an alibi; they could commit the robbery. Should I bring them in for questioning, sir?"