Continuing what Scott had to say:  3 things happen here to explain why Paul didn’t know who was tormenting him. (on the road to Damascus)

1. The light from heaven flashed all around him… so He fell to the ground and only heard a voice. His eyes were closed (v8)
2. The men around heard the sound but could not see anyone
3. Saul was struck blind… this is the important statement…. “But, when he opened his eyes, he could see nothing”

Acts 26:26 states that what Jesus spoke and did was out in the open for everyone to hear. (Paul was raised in Jerusalem.) Acts 22:3, Acts 21:17

Saul (Paul) was trained by Gamaliel and both men would have most probably been in the Temple and heard Jesus speak. 

In the Gospel to the Jews, Matthew states (in the end of Chapter 22 and thru 23) that Jesus addressed the Pharisees. Saul being a Pharisee would have been there and heard Jesus speak.

Deuteronomy 16:16,  Luke 22:1, both tell us about the Feast of Unleavened Bread which is the Passover--which is when Jesus taught in Jerusalem.

Deuteronomy 16:16 New International Version (NIV) Three times a year all (Jewish) men must appear before the Lord your God at the place he will choose: at the Festival of Unleavened Bread, the Festival of Weeks and the Festival of Tabernacles. No one should appear before the Lord empty-handed. (So all Jewish men would have been there.)  

The point is, Paul and Jesus crossed paths, probably many times.  But Paul didn't recognize Jesus on the road to Damascus because he thought Jesus was dead, and because Paul was blinded.  But when Jesus identified himself, Paul realized that he had been killing the followers of the Promised Messiah--Jesus.  Paul was immediately convicted and ready to follow the living Christ--Jesus.  Paul never looked back.  He gave everything he was and knew to spread the Gospel.