[Pilsner]

If they weren't suspects yet, of all of the millions of cell phones, how did the authorities just happen to track these two down? Maybe someone familiar with this can answer the questions.

The police get a judge to order the cell tower operators to disclose the 10 digit number of every cell phone that entered a small area around each site, during a brief window of time during which each attack occurred. When that is produced the police look for patterns, such as two cell phones being at all four remote locations in the wee hours of the morning, just when the attacks occurred. The police then get an order from a judge for the cell service provider to turn over the ID of the owner, call history and cell history of the two cell phones in question.

[Pilsner]

Text message history, not cell

[pfflier]

Thank you, very informative.

Question now, , do the cell phones have to be in use or simply be on for this type of search to be effective?

On the surface it sounds like the tracking mode has to be on in some manner and/or the user has to make a call.