stolen 2014 Diamondback Insight

I locked my bike to the bike rack near the escalator on the College Ave side of the Rockridge Bart. I put it towards the middle of the rack using a smaller Kryptonite U-lock and cable. I placed the U-lock through the front wheel, including the frame (I believe) and connected the cable through the back wheel, to the U-lock. My helmet was also attached to the U-lock. I left my bike at approximately 7:45 AM. When I returned at 8:00 PM, my bike was gone. The U-lock was still locked to the rack and cable lay next to the rack, cut in half. The helmet, which had been secured to the U-lock, was lying next to the rack, unattached. I normally place my bike on a rack in the area five days a week.