The number of so-called pen register and trap-and-trace orders obtained by federal law enforcement agencies has increased 361 percent between 2009 and 2011, the ACLU said. The U.S. Department of Justice released the data to the ACLU after the civil rights group sued the agency under the Freedom of Information Act.

Pen registers capture outgoing data from a surveillance subject, while trap-and-trace orders capture incoming data, including the addresses of email messages who the subject is talking with on instant messages. The two types of surveillance are not supposed to record the contents of conversations.

Including the targets of telephone surveillance, "more people were subjected to pen register and trap-and-trace surveillance in the past two years than in the entire previous decade," Naomi Gilens, a legal assistant with the ACLU's Speech, Privacy, and Technology Project, .

U.S. law enforcement agencies obtained about 250 pen register orders for email and Internet communications in 2009 and about 200 trap-and-trace orders, the ACLU said. In 2011, U.S. agencies received more than 800 of each order.

A DOJ spokesman noted that a federal judge authorizes each pen register and trap-and-trace order. "As criminals increasingly use new and more sophisticated technologies, the use of orders issued by a judge and explicitly authorized by Congress to obtain noncontent information is essential for federal law enforcement officials to carry out their duty to protect the public and investigate violations of federal laws," said spokesman Dean Boyd.