Despite a growing population and a down economy, Prince William County’s crime rate dropped in 2011, according to the department’s annual report released this week.

Violent crimes — including murders, rapes and assault — were down just over 20 percent, according to the report. Last year’s three murders were a significant drop from 2010’s nine murders. Property crimes, such as burglary, were also down nearly 7 percent.

Police Chief Charlie T. Deane said those numbers are reflective of a national trend. He also credited the department’s initiatives to build good relationships with the community that stress “how important it is for [the community] to do their part,” he said.

Deane said that those relationships become even more important when crime escalates. “This won’t continue to go down forever,” he said.

Deane also credited Prince William’s efficient, tough court system and commonwealth attorney’s office with helping to keep criminals at bay.

“Finding people guilty and the certainty of punishment certainly has an impact,” he said.

The 2011 crime report caps a year of some difficult, complex cases, police said. Last year, police captured a man they suspect is the East Coast Rapist, whose crimes began in Prince William on Halloween 2009 but “by the miracle of DNA” was linked to rapes in other East Coast states, according to Deane and authorities.

Police also apprehended a suspect in the killing of Kwang Sup Hur, a 75-year-old Woodbridge man who was found dead in September. One of the three suspects in that case was caught Sept. 10, ahead of 10-year memorial events for 9/11 in New York, at Newark Liberty International Airport. Police believed he was trying to flee the country.

“That was a case that started with no leads,” Deane said, and resulted in three arrests.

This year, Deane said he’s looking for new ways to focus on cases that involve computer crimes — including child pornography, sexual predators and identity theft. They are increasingly complex and the number of such cases continues to grow.

“It seems like a bottomless pit,” he said of those types of crimes.