BART issued a statement on Friday to explain the cell phone disruption in four downtown San Francisco stations Thursday, which stirred criticism by civil libertarians.

According to the statement published in BART Web site, the protesters were planning to disrupt BART service on Thursday, claiming that they would "use mobile devices to coordinate their disruptive activities and communicate about the location and number of BART Police."

The demonstration is for Charles Blair Hill, a homeless, who was shot to death on July 3 on the Civic Center station platform, after he threw a bottle at police and then approached a BART officer and displayed a weapon.

"A civil disturbance during commute times at busy downtown San Francisco stations could lead to platform overcrowding and unsafe conditions for BART customers, employees and demonstrators," BART's statement said.

To " to provide, safe, secure, efficient, reliable, and clean transportation services ," BART temporarily interrupted wireless service at four BART stations "as one of many tactics to ensure the safety of everyone on the platform."

BART emphasized that the cell phone service only interrupted inside of station, not outside the station. Even in the station, many radio of BART police and staff were operating during the interrupted period.

BART noted that the demonstration in the BART is illegal, and the areas for demonstrations have already been set down.

However, the American Civil Liberties Union still said interruption of cell phone was objectionable and expressed concern about passengers' safety.

"Shutting down access to mobile phones is the wrong response to political protests," the ACLU's Rebecca Farmer said in a blog.

Because of the interruption of cell phone, no demonstrations took place Thursday.

It wasn't the first protest for Hill's death. On July 11, the protest disrupted service during the rush-hour commute, prompting the closure of the Civic Center and ending in many arrests.

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