The number of robberies and motor vehicle thefts in Loudoun County increased sharply last year, but burglaries decreased nearly 19 percent.

The figures are included in a recently released report on crime in the Washington region. The report, prepared by the Metropolitan Washington Council of Governments, found that although the number of major crimes in the region barely changed from 2004 to 2005, robberies jumped nearly 19 percent and homicides were up 10 percent.

Law enforcement and public safety officials in the Washington area said that pinpointing the reasons that certain crimes fluctuate or become trends is difficult. But many crimes, especially robberies, are increasing because of a surge in development and population, a residual effect of Washington's economic growth, the officials said.

"Law enforcement is swimming against a strong tide and a strong economy," said David J. Robertson, executive director of the Council of Governments, a nonprofit group based in the District. "When you have communities that are growing in population, that means you're growing in population of criminals, too."

In Loudoun, one of the fastest- growing counties in the nation, the number of major crimes overall rose 4.6 percent, from 3,126 in 2004 to 3,271 in 2005. Robberies were up 11 percent, larcenies by 6.5 percent -- from 2,492 incidents in 2004 to 2,655 incidents in 2005 -- and motor vehicle thefts by more than 14 percent.

The county's population grew 6.8 percent from July 1, 2004, to July 1, 2005, according to census estimates.

"One of the factors we're dealing with is an affluent population," said Kraig Troxell, a spokesman for the Loudoun County Sheriff's Office. "And with that, you're going to see a lot of people coming into the area knowing that there are high-end cars with high-end items in them."

Thieves visit densely populated shopping areas with a checklist of what they want, Troxell said. Then they hop from vehicle to vehicle stealing items. "This is an area that's still very trusting, so we have people leaving iPods, purses and laptops in unlocked cars," he said.

In the summer, residents leave garage doors open at night, attracting criminals. "They forget to close it, and the next thing you know, bikes have been stolen or beers have been taken out of the garage refrigerator," Troxell said.

In bulging exurbs such as Loudoun and Prince William counties, law enforcement authorities also have reported that homes under construction can attract looters.

"Sometimes, the construction supplies are not secured. They are inviting targets for thieves," said Charlie Deane, police chief in Prince William County, which had a 2.1 percent decrease in crime despite its steady population growth.

Asked why burglaries in Loudoun were way down last year, Troxell said, "It's hard to put your finger on the reason, other than we've got an increased amount of deputies on the street."

Troxell said that in order to keep up with growth, the Sheriff's Office hopes to have one deputy per 1,000 residents. It now has 0.82 deputies per 1,000 residents. Efforts also include community policing, in which patrol officers work with community leaders and social service providers.

Regionwide, the number of serious offenses dropped 0.2 percent, although Howard, Anne Arundel, Charles, St. Mary's and Calvert counties were not included in the report.

The biggest percentage decrease in the number of major crimes occurred in the city of Fairfax, which had an 18.5 percent drop, from 823 incidents in 2004 to 671 in 2005.

Loudoun County and the city of Manassas were the only jurisdictions in Northern Virginia where overall crime increased.

Robertson, the council's executive director, said that part of the challenge for law enforcement officials is that the Washington area is one of the fastest-growing areas in the country. "It's difficult for communities that are growing fast," he said.