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"We're very concerned by reports that several Pakistani bloggers and activists have been reported missing and we're going to continue to monitor the situation," State Department Deputy Spokesman, Mark Toner, told reporters at his daily news conference.
At the same time, the State Department welcomed the Pakistani interior ministry's announcement that it would investigate the disappearance of one of those individuals, Salman Haider.
"We also appreciate the fact that members of both houses of parliament have voiced their concern and called for an investigation into all four disappearances," Toner said.
"Of course, we value freedom of expression. That's something we take very seriously and are going to continue to monitor the situation in Pakistan," the Deputy Spokesman said.