A House of Representatives panel called the hearing on the Geolocational Privacy and Surveillance Act, aimed at protecting people from being tracked by police through their phones without a search warrant, except in emergencies.

Catherine Crump of the American Civil Liberties Union welcomed the proposal and said it would help ensure constitutional rights against "unreasonable" searches.

"Americans' privacy rights are threatened by warrantless access to geolocational information, and history teaches that the executive cannot be counted upon to police itself," she told the committee.

The ACLU maintains that many police forces take advantage of the ambiguity in the law to track people without consent or probable cause of a crime.

"Congress cannot afford to wait any longer to enact a warrant and probable cause requirement for location tracking," she said. "Today Americans' privacy rights are being violated routinely by invasive location tracking, particularly cell phone tracking."

But John Ramsey of the Federal Law Enforcement Officers Association said the law would tie the hands of police when they are investigating crimes or trying to catch criminals.

"Who are we protecting with this legislation? The innocent or the criminals?" he said.

"Do we really want to slow down the apprehension of murderers and rapists so they can build their trophy wall by increasing the amount of legal documents necessary to gather information?"

Joseph Cassilly of the National District Attorneys Association said the law "would hamper law enforcement's ability to quickly obtain important information that could be used to save lives."

"Because so many cases are time sensitive in nature -- including child abductions, other forms of kidnapping and organized criminal and/or terrorist activities -- law enforcement must be able to work these cases without unnecessary administrative delay," Cassilly said.

But Ed Black of the Computer & Communications Industry Association backed stronger privacy protection, saying, "To cede to government the unchecked power to track you wherever you are is to lay the cornerstone of the surveillance state."

The US Supreme Court has held that the use of GPS devices placed by police on a suspect's car constitutes an "unreasonable search" under the constitution. But the question of cell phone tracking is still making its way through the courts.

Several members of Congress have introduced bills calling for "location privacy" to be respected by police, except in cases of emergency.

Privacy advocates say real-life police, like the ones on TV, often use phones to track suspects despite the murky legal situation. This could be limited if courts provide clearer rulings or if Congress passes legislation on "location privacy."

Here are examples, some drawn from "Best of Facebook Stories," a compilation of anecdotes that appears -- where else -- on Facebook, and others from the news media:

FATHER: Jessica Mayer always knew she'd been adopted. Her biological father's name was on her birth certificate, but for years, the American could never seem to find him -- until a friend spotted the dad's profile on Facebook with his surname spelled differently. Father and daughter went on to be reunited in Britain. (Facebook)

SAFE: Worried about her son's strangely puffy face, New York mother Deborah Copaken Kogan posted photos of the four-year-old boy on Facebook after doctors failed to identify the problem. A friend immediately recognized a rare case of Kawasaki disease, an inflammation of the blood vessels, called the mother and told her to get the lad to hospital. His life was saved. (msnbc.com)

SURVIVORS: No sooner did US Airways Flight 1549 splash into the Hudson River in New York than a Facebook page was created to enable the 155 survivors (there were no fatalities) to share news, photos and good wishes at every anniversary of the incident. Two passengers who sat three rows apart on the aircraft befriended themselves on Facebook, and went on to fall in love. (Facebook)

CLEAN-UP: Forty-eight hours after August 2011 riots in London flared, a "Post riot clean-up: Let's help London" page was set up to urge Facebook users to take up their brooms and clean debris off the streets. It gathered 3,600 users in a matter of hours, of whom hundreds heeded the call. (Facebook).

LEUKEMIA: US teenager Samir Pendse urgently needed a bone marrow transplant that could only come from someone who shared his South Asian ethnicity, but few persons who could fit that requirement were registered marrow donors. Out of a Facebook campaign to find a life-saving donor for Pense, Stanford University marketing professor Jennifer Aaker developed 100K Cheeks, a vast database of potential South Asian donors who could step forward and help in similar medical crises. (Stanford University News)

BIGAMY: Alan L. O'Neill, a 41-year-old from Washington state, left his wife without getting a divorce, then remarried under another name. On Facebook, wife number one befriended wife number two, who had proudly posted a photo of her new husband and their wedding cake. The result: O'Neill found himself facing bigamy charges in court. (The Huffington Post)

REVOLUTION: In the midst of anti-government protests in Tunisia, one "Ali" spent 18 hours a day overseeing a Facebook page filled with new photos and videos from demonstrators. Despite a crackdown that targeted Internet users in particular, president Zine El Abidine Ben Ali fled the country in mid-January 2011. (The Daily Beast)