U.S. homeowners entered the foreclosure process in April at more than double the rate of a year ago as tightening credit made it more difficult to refinance and a swelling supply of unsold homes made it tough to sell.

The number of homeowners in all three phases of foreclosure rose last month over the same period a year ago, according to Sacramento, Calif.-based Foreclosures.com, which gathers data from county courthouses nationwide.

Those receiving their first notice of foreclosure from a bank climbed 127 percent, those with homes going up for sale by auction jumped 164 percent and those whose homes were repossessed by banks went up 40 percent.

Eight of 10 subprime loans, given to borrowers with bad or limited credit histories, adjust over time to higher interest rates. Many homeowners can no longer afford their mortgages.

With existing-home sales at a four-year low, it's more difficult to sell because there are so many homes on the market.

"The housing boom was a house of cards," said Alexis McGee, president of Foreclosures.com. "A lot of people who are living beyond their means and borrowing from Peter to pay Paul find that it's starting to catch up with them. We're seeing the effects of aggressive lending and minimal standards for underwriting."

The number of foreclosure filings decreased in April in all three categories compared with March, Foreclosures.com said. Notices of default dropped 16 percent, auctions decreased 12 percent and bank repossessions fell 14 percent.