Tracy Rodriguez of Houston, Texas, is not a trained private investigator or police officer. But with a gentle tap on her iPhone screen, the mother of three can access information revealing the sex offenders who live within a 10-mile radius of where her children practice sports or watch movies.

Tracy Rodriguez, who is holding the iPhone, uses the device to locate sex offenders.

The information, provided by an iPhone app called Offender Locator, helps Rodriguez make more informed choices, she says. When the app pops open on her phone, there is an eerie sketch of a man's face. Then, the app asks for an address.

"I am constantly worrying about the well-being of my family," says Rodriguez, who uses the app several times daily. "You can't be too careful."

Since the iPhone launched more than two years ago, a handful of crime-fighting tools have emerged among the thousands of innovative apps. They give ordinary citizens the capability to sleuth and guard themselves against crime. Users can conduct a background check during a dinner date or avoid walking through a high-crime area.

The Offender Locator app has been downloaded more than a million times, breaking into the top 10 most popular apps list on iTunes when it made its debut in June.

Some BlackBerry models and Google's Android also offer crime-fighting apps. And the app industry is bound to grow, which probably means more inventive tools to fight crime. The Yankee Group Research Inc., a company with expertise in global connectivity, estimates U.S. smartphone app downloads will reel in $4.2 billion in revenue by 2013.

For the past decade, law enforcement agencies have relied on e-mail and texting to interact with the public. Now, iPhone apps are expanding their reach by allowing people to access information wherever they are, as long as they have cell phone service.

"I think mostly it will be used by users who want to do their own little reporting," says MG Siegler, a writer at TechCrunch.com, a blog about technology start-ups. "These apps are definitely very popular."

In February, the FBI worked with NIC Inc., a contractor that develops Web pages for the government, to construct an app that provides updates of the 10 most wanted fugitives and terrorists. So far, there have been more 541,000 downloads in 170 countries since the app was released in February.

This month, Intelius Inc., a company that provides public records online, is launching an iPhone app called Date Check, which is "like having a private investigator in your purse," company officials say. During a blind date, the Date Check user can snoop right away by punching in the stranger's name. Within seconds, users can read records showing whether their date is a convicted rapist or owns a million-dollar property.

"It gives you peace of mind of who you might be interacting with," said John Arnold, executive vice president of Intelius.

But Lillie Coney, associate director at the Electronic Privacy Information Center, a public interest research group, warns that the crime-fighting apps can spread inaccurate and misleading information. Public records can have errors, and sometimes, court records may be expunged or sex offenders are removed from registries. Some apps carry disclosures that say the information may not be accurate or up-to-date.

"It's turning everyone into a police officer," she says. "Is that the way society's resources should be used?"

Most of the information available on these iPhone apps is already accessible online. For example, states require sex offenders to register and distribute their personal information on Web sites and in community notifications.

App makers say their features protect the public's safety by keeping individuals informed. The National Center for Missing & Exploited Children created the AMBER Alert app this year, which provides a real-time feed with details about the victim, abductor and vehicle. The app is programmed to let the user automatically send tips to the center.

Crime Spot, an iPhone app made by Excelltech Inc., generates maps of where crimes have occurred. This can help users, particularly travelers unfamiliar with a city, avoid particular routes or neighborhoods. Users can also report a crime. Another app, Citixen Me, created by JasonWeiner LLC, lets users police their communities by reporting drunk drivers and nuisances.

"They work for the agencies that are proactive and believe in crime prevention," says Mike Lopilato, head of Crime Watch USA, a nonprofit that provides emergency notifications through texts and e-mails for law enforcement agencies. "For those agencies into crime solving, they can send alerts and notify the public that something has happened."

But just how effective can the iPhone be in fighting crime?

Lopilato says the apps like Crime Spot, designed to guard the public from crime, instead could help criminals. They could use the information to avoid high-crime areas where police may be waiting.

Michael McCauley, founder of Iphoneappreviews.net, says people may give false crime reports, which would undermine the goal of the app. "The thing is there needs to be some way to rate the credibility of the reports being submitted," McCauley says.

In Montgomery County, Maryland, police are worried about PhantomALERT, an iPhone app expected to be released in the next few months that warns drivers about DUI checkpoints. To use this app, invented by Joe Scott, the user downloads information from the company's Web site that specifies where checkpoints, red-light cameras and speeding traps are. For about $10 a month, the phone will send out an audio alert to the driver to help dodge tickets.

"That is a risk to public safety, allowing a potentially impaired driver to avoid detection and possibly harm him or herself or someone else on the roadway," says Lucille Baur, a spokeswoman for the police department.

But Baur admits the app's ability to warn drivers about red lights or speeding zones could help drivers slow down.

A few weeks ago in Pittsburgh, Pennsylvania, an iPhone helped a robbery victim retaliate. Robbers snatched the victim's wallet, credit cards and iPhone, which was equipped with a global positioning function.

Using a computer, the victim was able to trace his phone to a nearby Walmart, and then to a restaurant. It wasn't long before police detained the suspects at a gas station and recovered the stolen items.