As communities push to turn themselves into massive wireless hotspots, unsuspecting Internet users are stumbling directly onto hacker turf, giving computer thieves nearly effortless access to their laptops and private information, authorities and high-tech security experts say.

It's an invasion with a twist: People who think they are signing on to the Internet through a wireless hotspot might actually be connecting to a look-alike network, created by a malicious user who can steal sensitive information, said Geoff Bickers of the FBI's Los Angeles cyber squad.

It is not clear how many people have been victimized, and few suspects have been charged with Wi-Fi hacking. But Bickers said that over the past couple of years, these hacking techniques have become increasingly common and are often undetectable. The risk is especially high at cafes, hotels and airports, busy places with heavy turnover of laptop users, authorities said.

"Wireless is a convenience, that's why people use it," Bickers said. "There's an axiom in the computer world that convenience is the enemy of security. People don't use wireless because they want to be secure. They use wireless because it's easy."

For Mark Loveless, it was a just a single letter of the alphabet that separated security from scam.

Logging on to his hotel's free wireless Internet in San Francisco last month, Loveless had two networks to choose between on his laptop screen — same name, one beginning with a lowercase letter, one with a capital. He chose the latter and, as he had done earlier that day, connected. But this time, a screen popped up asking for his log-in and password.

Loveless, a 46-year-old security analyst from Texas, immediately disconnected. A former hacker, he knew an attack when he saw one, he said.

Most Internet users do not.

About 14.3 million U.S. households use wireless Internet, and that figure is projected to grow to nearly 49 million households by 2010, according to JupiterResearch, which specializes in business and technology market research.

"There's literally probably millions of laptops in the U.S. that are configured to join networks named Linksys or D-Link when they are available," said Corey O'Donnell, vice president of marketing for Authentium, a company that provides security software. "So if I'm a hacker, it's as easy as setting up a network with one of those names and waiting for the fish to come."

Linksys and D-Link are two of the many commercial brands of wireless routers, products that connect a user to the Internet using radio frequency.

More than 300 municipalities across the United States are planning or already operating Wi-Fi service. Google and Earthlink plan to bring wireless access to all of San Francisco.

A September survey at Chicago's O'Hare Airport by Authentium revealed 76 peer-to-peer networks, or access points that are connected to via another user's computer, with 27 advertising access to free Wi-Fi — a trademark term for the technical specifications of wireless local area network operation. The company also found three networks had fake or misleading addresses, a sign that these hotspots could be hacker sites.

"At a busy place like O'Hare, in one hour a bad guy could get 20 laptops to connect to his network and steal the users' account information," said Ray Dickenson, an Authentium vice president.

Corporate networks are sometimes the most vulnerable, as employers push for a more mobile work force without always educating its users on the security risks of wireless Internet. "Once they've (hackers) got a toehold in a network, it's pretty much game over," Bickers said.