Comment by hornokplease

Another well-sourced story quotes BART Deputy Chief Benson H. Fairow as saying that the "flip was switched" so to speak under his authority, based on the contract that BART has with its wireless providers:

According to Fairow, the statement from BART saying that cell carriers participated in the shutoff isn't precisely it. Fairow says that language in BART's contract with carriers allows BART to "flip a switch" to cut off service for issues of safety. A spokesperson from one of SF's major wireless carriers confirms this, and says that the carriers themselves weren't involved in the shutdown.

"Ultimately, I'm the one that implemented it," said Fairow. "It was certainly run through through channels. A lot of thought went into this."

Source: http://sfappeal.com/news/2011/08/bart-cell-fcc.php

All of the reports that I have seen (including the CNet story linked above) have retracted any statement that the wireless carriers turned off the signal at BART's request.