WASHINGTON (AP) — Families crowding together into single homes because of the bad economy helped increase the number of homeless families last year, even as the total number of homeless individuals dropped, federal housing officials reported Wednesday.

Roughly 170,000 families spent at least one night in a shelter in 2009, up from 159,000 families in 2008 and 131,000 in 2007, according to the Housing and Urban Development Department's annual report to Congress released Wednesday. That increase, the report said, "is almost certainly related to the recession."

"It may be that many families already at risk of becoming homeless lacked sufficient support networks and became homeless almost immediately after the economy turned down," the report said. "A much larger group turned to family and friends and may be doubled up and still at great risk of becoming homeless."

HUD attributed the 2009 increase in family homelessness to the use of emergency shelters, rather than transitional housing. Homeless families stayed in shelters an average of 36 nights in 2009, up from 30 nights the year before, the report said.

"Not only did family homelessness continue to increase ... it also seems to have become more severe in the sense that it took the typical family longer to leave shelter," the report said.

HUD defined a family as a household with at least one adult and one child.

HUD Assistant Secretary Mercedes Marquez said the trend of families crowding together is expected to continue.

The Obama administration plans to unveil at the White House next week a national strategy to combat homelessness, with a goal of ending the problem, Marquez told reporters in a conference call.

"We are closer than ever to that," Marquez said.

Overall, 1.56 million people spent at least one night in emergency shelters or transitional housing, the report said. One-third of those individuals were part of a homeless family.

Volunteers counted 643,000 homeless people, sheltered and on the streets, during one given night in January 2009, the report said.

Chronic homelessness dropped 30 percent since 2006, which the report said may be due to an increase in resources.

The number for homeless families jumps in the summer as the school year ends. More individuals seek shelter during the winter.

Next year's report will show the impact of the $1.5 billion in homelessness assistance allocated under the economic stimulus, Marquez said.