[Update 1: clarifies homebuyer tax credit eligibility dates.] The Standard & Poor's/Case-Shiller index showed a 1% gain in June home prices, compared to a month earlier. The price of single-family homes in the 20-city composite index showed a 4.2% gain in June from the year ago. Overall, prices increased in 17 of those markets. The federal homebuyer tax credit expired at the end of April, but revisions to the programs allowed sales that close by Sept. 30 to be eligible. The S&P/Case-Shiller indices measure U.S. residential sales that have closed. S&P said second-quarter home prices were up 4.4% from the year-ago period and U.S. home prices are now 3.6% higher than year-earlier levels. "Housing prices have rebounded from crisis lows, but other recent housing indicators point to more ominous signals as tax incentives have ended and foreclosures continue," analysts said the monthly report. The 10-city composite index rose 5% in June from a year ago, but analysts warned of "a possible deceleration in home price returns" because June figures fell month over month for the first time in 16 months. In May, the 10-city composite rose 5.4% and the 20-city composite increased 4.6% from the year earlier. "The monthly composites cover June and the national index covers the second quarter, when the government’s program for first time home-buyers was winding down. While the numbers are upbeat, other more recent data on home sales and mortgages point to fewer gains ahead," said David Blitzer, chairman of the Standard & Poor's index committee. "Even with concerns about near term developments, we recognize that the housing market is in better shape than this time last year." While 17 metropolitan-statistical areas had monthly gains in home prices, for June, Las Vegas fell 0.6% and Phoenix and Seattle remained flat with May. S&P said three quarters of the 20 MSAs have positive annual growth rates, and no market is down double digits. Las Vegas has the steepest decline with a 5.2% drop from a year earlier. And three MSAs have experienced double-digit gains in home prices from the year ago: San Francisco 14.3%, San Diego 11.2%, and Minneapolis 10.7%. "The worry starts when you remember that the homebuyers’ tax credit has expired, foreclosures are still at high levels, and July data on home sales and starts were very, very weak," Blitzer said. "The inventory of unsold homes and months’ supply data were particularly troubling. If this relative weakness in demand continues, it will likely filter through to home prices in coming months." Blitzer may have good reason to sound cautious. Other well-known HPI providers, including Radar Logic, have suggested that U.S. home prices are already on the downward swing again, citing differences in methodology between the two indices. July homes sales fell 27.2% from the prior month to the lowest level since 1985. Write to Jason Philyaw.