WASHINGTON — When FBI investigators probing New York prostitution rings, Boston organized crime, or potential terrorist plots anywhere want access to a suspect’s telephone contacts, technicians at a telecommunications carrier served with a government order can, with the click of a mouse, instantly transfer key data along a computer circuit to an FBI technology office in Quantico, Va.

The circuits — little-known electronic connections between telecom firms and FBI monitoring personnel around the country — are used to tell the government who is calling whom, along with the time and duration of a conversation and even the locations of those involved.

Recently, three Democrats on the House Energy and Commerce Committee, including Chairman John Dingell, a Democrat of Michigan, sent a letter to colleagues citing privacy concerns over one of the Quantico circuits and demanding more information about it. Anxieties about whether such electronic links are too intrusive form a backdrop to the continuing congressional debate over modifications to the Foreign Intelligence Surveillance Act, which governs federal surveillance.

Since a 1994 law required telecoms to build electronic interception capabilities into their systems, the FBI has created a network of links between the nation’s largest telephone and Internet firms and about 40 FBI offices and Quantico, according to interviews and documents describing the agency’s Digital Collection System. The documents were obtained under the Freedom of Information Act by the Electronic Frontier Foundation, a nonprofit advocacy group in San Francisco that specializes in digital-rights issues.

The bureau says its budget for the collection system increased to $40 million in 2008 from $30 million in 2007. Information lawfully collected by the FBI from telecom firms can be shared with law enforcement and intelligence-gathering partners, including the National Security Agency and the CIA. Likewise, under guidelines approved by the attorney general or a court, some intercept data gathered by intelligence agencies can be shared with law enforcement agencies.

“When you’re building something like this deeply into the telecommunications infrastructure, when it becomes so technically easy to do, the only thing that stands between legitimate use and abuse is the complete honesty of the persons and agencies using it and the ability to have independent oversight over the system’s use,” said Lauren Weinstein, a communications systems engineer and co-founder of People for Internet Responsibility, a group that studies Web issues. “It’s who watches the listeners.”

Different versions of the system are used for criminal wiretaps and for foreign intelligence investigations inside America. But each allows authorized FBI agents and analysts, with point-and-click ease, to receive e-mails, instant messages, cell phone calls, and other communications that tell them not only what a suspect is saying, but where he is and where he has been, depending on the wording of a court order or a government directive. Most of the wiretapping is done at field offices.

Wiretaps to obtain the content of a phone call or an e-mail must be authorized by a court upon a showing of probable cause. But “transactional data” about a communication — from whom, to whom, how long it lasted — can be obtained by simply showing that it is relevant to an official probe, including through an administrative subpoena known as a national security letter. According to the Justice Department’s inspector general, the number of NSLs issued by the FBI soared to 47,000 in 2005 from 8,500 in 2000.

The administration has proposed expanding the types of data it can get from telecom carriers under the 1994 Communications Assistance for Law Enforcement Act, so FBI agents can gain faster and more detailed access to information sent by wireless devices that discloses where a person is in real time. The Federal Communications Commission is weighing the request.

“Court-authorized electronic surveillance is a critical tool in pursuing both criminal and terrorist subjects,” an FBI spokesman, Richard Kolko, said.

A Justice Department spokesman said the government is asking only for information at the beginning and end of a communication, and for information “reasonably available” in a carrier’s network.

A telecom industry lawyer at Perkins Coie in Seattle who handles wiretap orders for companies, Al Gidari, said government officials now “have to rely on a human being at a telecom calling up every 15 minutes to send law enforcement the data.”