Police started looking at other recent street robberies reported in the area. It turned out that a week before the murder, a person was robbed at gunpoint just a few blocks from where the murder took place, and their cellphone was taken. After that theft, call records for that stolen phone started showing showed a woman's phone number.
Upon tracking down that number, they got to a woman who said her boyfriend was involved in the murder. That led to two men being arrested for this and at least one other previously unsolved armed robbery.
http://www.stltoday.com/news/local/metro/cellphone-played-bi...
However, the NYT article says it can take weeks for a subpoena to be issued and acted upon by the phone company. So I question whether a database like that would really have even been helpful in a situation like this, since they'd need the data a lot sooner after an incident.
Seems to me that this might be more of an attempt to mine phone records on a larger scale and look for drug networks or other emerging patterns.