Interim General Manager Sherwood Wakeman and board of directors President Bob Franklin signed the letter, which gave BART customers a broad overview of this summer's events, which began with the BART police shooting of Charles Hill on July 3.
A week later, on July 11, a group gathered on the platform at Civic Center station, where Hill was shot, and disrupted service by blocking train doors, climbing on top of trains, and moving from station to station to stop trains from leaving.
In response, BART closed three San Francisco BART stations and commuters were forced to make other travel arrangements. BART's letter said that 96 trains were disrupted at the height of rush hour.
The protests were organized by a group called "No Justice, No BART" which formed after the Jan.1, 2009 BART police shooting of Oscar Grant in Oakland.
In August, BART announced on its website that it had obtained information that another protest was being planned for Civic Center station on Aug. 11. According to Saturday's letter, the demonstration was being planned as a surprise, with different color-coded groups coordinating activities to disrupt the trains via cellphones.
"The overall information about the planned protest led BART to conclude that the planned action constituted a serious and imminent threat to the safety of BART passengers and personnel and the safe operation of the BART system, at a level that could far exceed the protest of July 11," the letter said.
To prevent protesters from coordinating, BART suspended cellphone service in BART stations and tunnels beginning at 4 p.m. Aug. 11. The protests never materialized, leading some BART officials to declare the precaution was successful.
But the move set off a flurry of criticism and a fresh round of protests. Public statements from the Electronic Frontier Foundation and the American Civil Liberties Union compared BART's strategy to those employed by repressive regimes in Egypt and Libya, where cellphone and Internet disruption is a standard practice in abating protests.
In addition, the loosely organized hacker protest group "Anonymous" reacted strongly to BART's strategy, calling for a new protest and hacking the BART website myBART.org, releasing some customers' personal information to the Internet.
The Anonymous-organized protest on Aug. 15 again met on the Civic Center platform and prevented trains from leaving the station, which led to the closure of all four downtown San Francisco BART stations.
The same day, the Northern California ACLU released another statement, declaring BART's decision to shut down cellphone service unconstitutional, and the Federal Communications Commission released a statement saying it was gathering information about the unusual strategy to determine if any regulations had been broken.
Within BART itself there has been controversy about whether disrupting cellphone service was an appropriate measure. Board member Lynette Sweet said on Tuesday that she thinks the board should have been consulted before such a decision was reached, and said Wakeman himself lacks accountability.
"What we ended up doing is giving these same people another reason to come back and protest us," Sweet said.
On Wednesday, another BART website was hacked and personal information of BART police officers was released to the Internet. While Anonymous lacks an official head or spokesperson, Twitter accounts and other websites speaking for the group have denied responsibility for the latest leak, and some have condemned it.
Still, the group functions without any formal registration or membership, so anyone who chooses to call themselves Anonymous can do so. One Twitter account clarified this aspect of the group by saying, "Anonymous is not unanimous."
Anonymous has called for a second protest on Monday at 5 p.m., and as of 11 a.m. today, 215 people said they would be attending on a Facebook page announcing the action, even more than said they would be attending last week's protest.
The page said the protest would again meet on the platform at the Civic Center BART station.
BART officials have said that protests are permitted outside the faregates in BART stations, but that any protest in paid areas would be illegal. While BART has not shut down cell service since Aug. 11, BART officials have maintained that the move was legal and has not ruled out that they would again take that step during future protests.
The BART board of directors is scheduled to discuss the issue during a special meeting on Wednesday at BART's headquarters in Oakland, and has invited any concerned members of the public to attend.