Phone companies know where their customers' cellphones are, often within a radius of less than 100 feet. That tracking technology has rescued lost drivers, helped authorities find kidnap victims and let parents keep tabs on their kids.

But the technology isn't always used the way the phone company intends.

One morning last summer, Glenn Helwig threw his then-wife to the floor of their bedroom in Corpus Christi, Texas, she alleged in police reports. She packed her 1995 Hyundai and drove to a friend's home, she recalled recently. She didn't expect him to find her.

Marsie Silvestro, who runs battered-women shelters, says tracking is a problem. John Tully for The Wall Street Journal

The day after she arrived, she says, her husband "all of a sudden showed up." According to police reports, he barged in and knocked her to the floor, then took off with her car.

The police say in a report that Mr. Helwig found his wife using a service offered by his cellular carrier, which enabled him to follow her movements through the global-positioning-system chip contained in her cellphone.

Mr. Helwig, in an interview, acknowledged using the service to track his wife on some occasions. He says he signed up for the tracking service last year. "AT&T had this little deal where you could find your family member through her cellphone," he says. But he didn't use it to find his wife that day, he says. Mr. Helwig, who is awaiting trial on related assault charges, declined to comment further about the matter. He has pleaded not guilty.

Technology is enhancing the reach of stalkers, allowing them to take advantage of location-based social networking applications. WSJ's Andy Jordan reports.

The allegations are a stark reminder of a largely hidden cost from the proliferation of sophisticated tracking technology in everyday life—a loss of privacy.

Global-positioning systems, called GPS, and other technologies used by phone companies have unexpectedly made it easier for abusers to track their victims. A U.S. Justice Department report last year estimated that more than 25,000 adults in the U.S. are victims of GPS stalking annually, including by cellphone.

In the online world, consumers who surf the Internet unintentionally surrender all kinds of personal information to marketing firms that use invisible tracking technology to monitor online activity. A Wall Street Journal investigation of the 50 most-popular U.S. websites found that most are placing intrusive tracking technologies on the computers of visitors—in some cases, more than 100 tracking tools at a time.

The cellphone industry says location-tracking programs are meant to provide a useful service to families, and that most providers take steps to prevent abuse. Mike Altschul, chief counsel for wireless-telecommunications trade group CTIA, says recommended "best practices" for providers of such services include providing notification to the person being tracked.

Mr. Helwig's wife had received such a notification, by text message, from AT&T. A spokesman for AT&T Inc. says it notifies all phone users when tracking functions are activated. But users don't have the right to refuse to be tracked by the account holder. Turning off the phone stops the tracking.

Cellphone companies will deactivate a tracking function if law-enforcement officials inform them it is being used for stalking. Mr. Altschul says authorities haven't asked carriers to change their programs. He adds that carriers have long supported programs to give untraceable cellphones to domestic-violence victims.

In Arizona this year, Andre Leteve used the GPS in his wife's cellphone to stalk her, according to his wife's lawyer, Robert Jensen, before allegedly murdering their two children and shooting himself. Mr. Jensen says Mr. Leteve's wife, Laurie Leteve, didn't know she was being tracked until she looked at one of the family's monthly cellphone bills, more than 30 days after the tracking began. Mr. Leteve, a real-estate agent, is expected to recover. He has pleaded not guilty to murder charges, and is awaiting trial. The law firm representing him declined to comment.

In a suspected murder-suicide last year near Seattle, a mechanic named James Harrison allegedly tracked his wife's cellphone to a store. After he found her there with another man, he shot to death his five children and himself, according to the Pierce County Sheriff's Office.

Therapists who work with domestic-abuse victims say they are increasingly seeing clients who have been stalked via their phones. At the Next Door Solutions for Battered Women shelter in San Jose, Calif., director Kathleen Krenek says women frequently arrive with the same complaint: "He knows where I am all the time, and I can't figure out how he's tracking me."

In such cases, Ms. Krenek says, the abuser is usually tracking a victim's cellphone. That comes as a shock to many stalking victims, she says, who often believe that carrying a phone makes them safer because they can call 911 if they're attacked.

There are various technologies for tracking a person's phone, and with the fast growth in smartphones, new ones come along frequently. Earlier this year, researchers with iSec Partners, a cyber-security firm, described in a report how anyone could track a phone within a tight radius. All that is required is the target person's cellphone number, a computer and some knowledge of how cellular networks work, said the report, which aimed to spotlight a security vulnerability.

The result, says iSec researcher Don Bailey, is that "guys like me, who shouldn't have access to your location, have it for very, very, very cheap."

That is, in part, an unintended consequence of federal regulations that require cellphone makers to install GPS chips or other location technology in nearly all phones. The Federal Communications Commission required U.S. cellular providers to make at least 95% of the phones in their networks traceable by satellite or other technologies by the end of 2005. The agency's intention was to make it easier for people in emergencies to get help. GPS chips send signals to satellites that enable police and rescue workers to locate a person.

To a large extent, that potential has been fulfilled. Last year, for example, police in Athol, Mass., working with a cellphone carrier, were able to pinpoint the location of a 9-year-old girl who allegedly had been kidnapped and taken to Virginia by her grandmother. In December, police in Wickliffe, Ohio, tracked down and arrested a man who allegedly had robbed a Pizza Hut at gunpoint by tracking the location of a cellphone they say he had stolen.

Mr. Altschul, of the cellphone-industry trade group, says the tracking technology has been of great help to both law-enforcement officials and parents. "The technology here is neutral," he says. "It's actually used for peace of mind."

But as GPS phones proliferated, tech companies found other uses for the tracking data. Software called MobileSpy can "silently record text messages, GPS locations and call details" on iPhones, BlackBerrys and Android phones, according to the program's maker, Retina-X Studios LLC. For $99.97 a year, a person can load MobileSpy onto someone's cellphone and track that phone's location.

A memorial near Seattle for five children murdered by their father, who then killed himself, after tracking his wife by cellphone. Courtney Blethen/The Seattle Times

Craig Thompson, Retina-X's operations director, says the software is meant to allow parents to track their kids and companies to keep tabs on phones their employees use. He says the company has sold 60,000 copies of MobileSpy. The company sometimes gets calls from people who complain they are being improperly tracked, he says, but it hasn't been able to verify any of the complaints.

Installing such programs requires a person to physically get hold of the phone to download software onto it.

GPS-tracking systems provided by cellular carriers such as AT&T and Verizon Communications Inc. VZ 0.42% are activated remotely, by the carriers.

Domestic-violence shelters have learned the consequences. As soon as victims arrive at shelters run by A Safe Place, "we literally take their phones apart and put them in a plastic bag" to disable the tracking systems, says Marsie Silvestro, director of the Portsmouth, N.H., organization, which houses domestic-violence victims in secret locations so their abusers can't find them.

The organization put that policy in place after a close call. On Feb. 26, Jennie Barnes arrived at a shelter to escape her husband, Michael Barnes, according to a police affidavit filed in a domestic-violence case against Mr. Barnes in New Hampshire state court. Ms. Barnes told police she was afraid that Mr. Barnes, who has admitted in court to assaulting his wife, would assault her again.

Ms. Barnes told a police officer that "she was in fear for her life," according to court filings. The next day, a judge issued a restraining order requiring Mr. Barnes to stay away from his wife.

Later that day, court records indicate, Mr. Barnes called his wife's cellular carrier, AT&T, and activated a service that let him track his wife's location. Mr. Barnes, court records say, told his brother that he planned to find Ms. Barnes.

The cellular carrier sent Ms. Barnes a text message telling her the tracking service had been activated, and police intercepted her husband. Mr. Barnes, who pleaded guilty to assaulting his wife and to violating a restraining order by tracking her with the cellphone, was sentenced to 12 months in jail. A lawyer for Mr. Barnes didn't return calls seeking comment.

Another source for cellphone tracking information: systems meant to help police and firefighters. Some cellular carriers provide services for law-enforcement officers to track people in emergencies. Using such systems requires a person to visit a special website or dial a hot-line number set up by the carrier and claim the data request is for law-enforcement purposes.

Cellular carriers say they try to verify that callers are legitimate. An AT&T spokesman says an office is manned around the clock by operators who ask for subpoenas from law-enforcement officials using the system.

But federal law allows carriers to turn over data in emergencies without subpoenas. Al Gidari, a lawyer who represents carriers such as Verizon, says such location-tracking systems can be easy to abuse. Police, he says, often claim they need data immediately for an emergency like a kidnapping, and therefore don't have time to obtain a warrant, in which a judge must approve an information request.

In Minnesota, Sarah Jean Mann claimed last year in a county-court petition for a restraining order that her estranged boyfriend, a state narcotics agent, followed her by tracking her cellphone and accessing her call and location records through such a system. The court issued the restraining order. The boyfriend, Randy Olson, has since resigned from the police force. He didn't respond to calls seeking comment.

Mr. Gidari says law-enforcement's easy access to such data makes the systems easy to abuse. He says carriers would like to have a system in place requiring agents to get warrants. Without such a requirement, there is little carriers can do to resist warrantless requests, say Mr. Gidari and Mr. Altschul of trade group CTIA. Federal law says carriers may comply with such requests, and law-enforcement agencies have pressured them to maintain the tracking systems, Mr. Gidari says.

The easiest way for stalkers to locate a target—and perhaps the most common, say therapists who work with victims and abusers—is by using systems offered by carriers. When cellphone users sign up for a "family plan" that includes two or more phones, they have the option to contact the carrier and activate a tracking feature intended to allow them to keep tabs on their children.

The AT&T FamilyMap program, for example, is free for 30 days and requires only a phone call to activate. "Know where your kids and loved ones are at any time!" says AT&T's website. The system is for parents, says an AT&T spokesman. He says the company hasn't received complaints about FamilyMap being used by stalkers.

The system provides an on-screen map on the smartphone or computer of the person doing the tracking. A dot on the map shows the location and movement of the person being followed. The carrier sends a text-message to the person being tracked that their phone is registered in the program.

These add-on services can be lucrative for carriers. AT&T debuted its FamilyMap system in April 2009. It charges $9.99 a month to track up to two phones, $14.99 for up to five. FamilyMap users must agree to "terms-of-use" stating that they may not use the system to "harrass, stalk, threaten" or otherwise harm anyone.

In Corpus Christi, Mr. Helwig and his wife, who had been married since early 2008, bought phones under an AT&T family plan. Mr. Helwig says he activated the feature last year. His wife says she received a text message that a tracking function had been activated on her phone, but wasn't sure how it was activated. Her husband, she says, initially denied turning on the tracking function.

Ignorance is NOT bliss and it is time for this to be fully debated out in the open. Stephen Babbitt Stephen Babbitt

She says she eventually came up with a plan to flee to the house of a family whose children she baby-sat. Her husband "had no idea where they lived" or even their names, she says. As she was packing, her husband confronted her. They argued, and, according to her statements in police reports, Mr. Helwig dragged her around by her hair.

The police came. She says she told them she didn't want them to arrest Mr. Helwig, that she simply wanted to leave. The police told Mr. Helwig to stay away from her for 24 hours, she says.

As she drove to her friend's house, she says, she made sure her phone was off so Mr. Helwig couldn't track her. But she turned it on several times to make calls. The next day, Mr. Helwig was outside in a rage, according to police reports.

Mr. Helwig forced his way into the house, pushed her to the floor, took her car keys and drove away in her Hyundai, according to police reports.

Police arrested Mr. Helwig a short distance away. Mr. Helwig, a firefighter, is facing charges of assault and interfering with an emergency call. His trial is scheduled to begin this summer.

Mr. Helwig and his wife divorced, and she left Corpus Christi. She says she doesn't want to testify against him. She says she is more careful about trusting her cellphone now.

Write to Justin Scheck at justin.scheck@wsj.com