WASHINGTON (AP) -- There were 744,000 homeless people in the United States in 2005, according to the first national estimate in a decade.

A little more than half were living in shelters, and nearly a quarter were chronically homeless, according to the report Wednesday by the National Alliance to End Homelessness, an advocacy group.

A majority of the homeless were single adults, but about 41 percent were in families, the report said.

The group compiled data collected by the Department of Housing and Urban Development from service providers throughout the country. It is the first national study on the number of homeless people since 1996. That study came up with a wide range for America's homeless population: between 444,000 and 842,000.

Counting people without permanent addresses, especially those living on the street, is an inexact process. But the new study is expected to provide a baseline to help measure progress on the issue.

"Having this data brings all of us another step closer to understanding the scope and nature of homelessness in America, and establishing this baseline is an extremely challenging task," HUD Secretary Alphonso Jackson said. "Understanding homelessness is a necessary step to addressing it successfully."

HUD is preparing to release its own report on homelessness in the coming weeks, Jackson said. In the future, the department plans to issue annual reports on the number of homeless people in the U.S.

Some cities and states have done their own counts of the homeless, providing a mix of trends, said Nan Roman, president of the National Alliance to End Homelessness. For example, New York City and San Francisco have seen decreases, while the number of homeless in Washington, D.C., has increased, Roman said.

"In the last 12 to 18 months, the homeless population has essentially exploded in Philadelphia," said Marsha Cohen, executive director of the Homeless Advocacy Project, which provides free legal services to the homeless in Philadelphia. "We are seeing big increases in singles and families, both on the street and attempting to enter the homeless system."

"It's a whole influx of new people, and that's the really scary part," Cohen said.

In Columbus, Ohio, workers are scrambling to help an increasing number of people living under bridges and in wooded encampments near rivers and streams, said Barbara Poppe, executive director of the Community Shelter Board.

"We're very concerned about the health and well being of those people being out in the elements," Poppe said. "We had an encampment set on fire, and we had a woman struck by a train."

California was the state with most homeless people in 2005, about 170,000, followed by New York, Florida, Texas and Georgia, according to the report.

Nevada had the highest share of its population homeless, about 0.68 percent. It was followed by Rhode Island, Colorado, California and Hawaii.

"The driver in homelessness is the affordable housing crisis," Roman said. "If we don't do something to address the crisis in affordable housing we are not going to solve homelessness."

She said many of the chronically homeless have mental health and substance abuse problems. Others, she said, simply cannot afford housing.