Prosecutors filed charges Thursday against three people who allegedly conspired with homicide bombers in the attacks that killed 52 subway and bus passengers in London on July 7, 2005.

For the first time, authorities alleged that the plot's targets may have included London tourist attractions.

The three, who were arrested last month, are from the same area of West Yorkshire, England, as three of the four homicide bombers.

Those charged were Mohammed Shakil, 30, of Beeston, a suburb of Leeds; Sadeer Saleem, 26, of Beeston; and Waheed Ali, 23, who recently lived in London but was formerly from Beeston.

"The allegation is that they were involved in reconnaissance and planning," said Susan Hemming of the prosecutors' office.

"The search is not over," added Peter Clarke, head of the Metropolitan Police anti-terrorist unit, who said he expected that there would be more arrests in the case.

"I firmly believe that there are other people who have knowledge of what lay behind the attacks in July 2005, knowledge that they have not shared with us," he said. "In fact I don't only believe it, I know it for a fact. For that reason the investigation continues."

An initial court appearance for the three men is scheduled for Saturday.

The three were formally charged with conspiring between Nov. 1, 2004, and June 29, 2005, to cause explosions on London's transport network "and/or tourist attractions in London of a nature likely to endanger life or cause serious injury."