Paul Callan is a CNN legal analyst, a former New York homicide prosecutor and currently is counsel at the New York law firm of Edelman & Edelman PC, focusing on wrongful conviction and civil rights cases. Follow him on Twitter @paulcallan. The opinions expressed in this commentary are his own.

(CNN) When I was a homicide prosecutor in the Brooklyn District Attorney's office, my most challenging assignment was "riding" homicide cases. I'd get paged (often in the middle of the night) and a patrol car would pick me up and take me to the grisly scene of a recent murder.

If a suspect was in custody, it was my job to evaluate the evidence and authorize an arrest if one could be legally supported. I had to be careful because the case would be mine to prosecute before a jury. A mistake would put an innocent suspect in prison while the true murderer roamed free. The stakes were high.

Sometimes in cases of all kinds, as in the Carter Page matter currently at the center of the controversy over a GOP memo on FBI surveillance, a search or surveillance warrant requiring the approval of a judge is required to gather critical evidence.

I remember one night knocking on the door of an elegant Brooklyn Heights townhouse seeking such a warrant from a tough, respected judge at approximately one in the morning. I had tried many murder cases before this judge and I was hopeful she would grant a search warrant in my case. She looked a little angry when answering the door in her bathrobe with cigarette and scotch in hand.

After berating me for bothering her at such an hour, she scrutinized my warrant application. It was based on the observations of a drug addict who claimed to have seen the defendant return to his apartment gun in hand after the murder and then leave without the gun shortly thereafter. The addict was known to the cops and had provided reliable information in the past.

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