The number of Amber Alerts, public announcements of a child's abduction, is falling as police use them only for kids in the most danger.

The number of alerts fell from 275 in 2005 to 261 in 2006 and 227 last year, according to the National Center for Missing & Exploited Children, a nonprofit group that helps the Justice Department train police to use the system. This year's figure, 102 alerts for the first six months, suggests a continuing decline.

Police and researchers see various reasons, including more restrictive use of the alerts, which are broadcast by radio, TV, e-mail and electronic highway signs.

"We're making sure the criteria are met," says Cinda Lubich, Illinois' Amber Alert coordinator. At first, she says, the state issued them more broadly.

Criteria vary by state, and sometimes by locality, but most follow federal guidelines: The child is younger than 18 and believed to be in grave danger, the abduction is confirmed by police and there is enough information to be useful.

"It's a heart-wrenching process to make the call" about whether a case merits an Amber Alert, says Virginia State Police Lt. Pete Fagan, state Amber Alert coordinator. He says for cases that don't meet the criteria, a less widely distributed secondary alert is issued.

Fagan says Virginia wants to restrict Amber Alerts to the most serious cases so they get maximum public attention. "If we overuse the media," he says, "it will stop cooperating."

California Highway Patrol Lt. L.D. Maples, state Amber Alert coordinator, says California, too, is careful about issuing alerts because it does not want to "wear out" the system.

"As the system has matured, we expect that it will be used with greater precision, and this may affect the number of alerts issued," says the Justice Department's Jeffrey Sedgwick, the national Amber Alert coordinator.

The Amber Alert program began in the Dallas area following the 1996 murder of 9-year-old Amber Hagerman, who was abducted while riding her bike. It was set up in her honor but is also known as America's Missing: Broadcast Emergency Response.

The overwhelming majority of Amber alerts end with children being quickly and safely recovered, says Bob Hoever, the National Center for Missing & Exploited Children's associate director for training. "We have the eyes and ears of the public assisting us."

Hoever says the alerts deter or curtail some abductions. He says last year, 16 abductors said they surrendered a child because an alert was issued.

David Finkelhor, director of the Crimes Against Children Research Center at the University of New Hampshire, says the declining number of alerts reflects a decline in crimes against children.

Finkelhor's research, based on figures from the Department of Health and Human Services, shows that between 1992 and 2006, the number of substantiated cases of sexual abuse fell 53 percent and those of physical abuse fell 48 percent.