Brake Abs Warning Light problems of the 2008 Buick Lacrosse

The brakes locked up in two different situations where I was traveling at approximately 10 mph. I had the car's braking system checked after the first time. I was told the braking system looked fine. The first incident I was on the freeway in stop and go traffic. The second time I was on a bridge over a highway making a left turn and was traveling about 10 mph. Both times traffic stopped in front of me and I applied the brakes and all four wheels locked up and I slid into the cars in front of mine. I had no abs light on the dashboard and no indication my brakes were in distress.