I think it took longer because they were pre-paid cell phones.
Based on what Ben Levitan, a telecommunications expert said last nite on NG, LE could have gone straight to the phone companies for the records they needed and apprehending the phones was unnecessary.
Snip
GRACE: I want to go to Ben Levitan, telecommunications expert, joining us out of Raleigh. Ben, these phones were disposable phones. What does that mean? Will police be able to track them?
BEN LEVITAN, TELECOMMUNICATIONS EXPERT (via telephone): It doesn`t matter, Nancy. A prepaid phone, or like you call it, a disposable phone, is exactly the same as any other phone. It can be traced. It can be wiretapped. It makes no sense for the police to take the phones away. All the information they need is available from the phone companies, and that information for every person involved in this should be grabbed right now.
http://edition.cnn.com/TRANSCRIPTS/1101/04/ng.01.html