(CNN) -- Federal aid helped many cash-strapped Americans keep a roof over their heads during the prolonged economic slump, but the number of people living a step away from the streets has grown sharply, researchers reported Wednesday.
The estimated U.S. homeless population dipped about 1% between 2009 and 2011 despite the lingering effects of the 2007-2009 recession, the Washington-based Homelessness Research Institute concluded.
About $1.5 billion from the 2009 economic stimulus measure went toward rental assistance and programs steering recently evicted people toward new housing, "and it seems likely that that has worked," said Nan Roman, president of the National Alliance to End Homelessness.
Nevertheless, the homeless population went up in 24 states during the past two years, the Washington-based researchers found. And the number of people who have had to move in with friends or relatives -- often a last stop before the street -- is up as well, according to Wednesday's report.
The Homelessness Research Institute, the educational arm of Roman's organization, put the number of Americans living on the streets or in shelters at just over 636,000 in 2011. That's down about 6,000 from the group's 2009 estimate. The figure is based on reports and street counts from state and local agencies that receive federal housing funds.
Roman said the stimulus money, coupled with pre-recession federal programs aimed at veterans and the chronically ill, have kept that figure down even as the U.S. economy saw its worst downturn since the 1930s. But that money is drying up now that the Obama administration, Congress and the states are grappling with budget issues fueled by the recession.
"Just like all domestic discretionary spending, it's vulnerable at the moment because of the deficit," Roman said. States have already been cutting services, "So again, it's notable that these are still working."
The report found the effects of the recession keep cascading downward more than two years after its official end in mid-2009.
More than 4 million homes have been lost to foreclosure since 2007, according to the online marketer RealtyTrac. Those foreclosures have driven many ex-homeowners into the rental market, driving up rents and pushing some of the working poor to move in with family, friends or other acquaintances.
That "doubled-up" population grew by about 800,000 people between 2009 and 2010, the last year data was available, the HRI report concluded. About 6.8 million were in combined households in 2010, a more than 50% increase from 2005, researchers found.
"The most common previous address for homeless people is that they were doubled up with someone else," Roman said. The new figures are "not a good sign. It means we've got potentially a lot of people moving toward homelessness."
The number of low-income households that spend more than half their income on rent grew by 300,000 in 2009, to 6.2 million. That's about three-quarters of poor renters, the HRI study found.
But the figures showed slight declines among nearly all categories examined, including a more than 10% decline in the number of unemployed veterans and about a 3% dip in the ranks of the chronically homeless -- people with disabilities like severe mental illness or addiction that result in repeated or extended stays on the street.