Federal and state authorities are reporting a growing number of financial scams that echo the alleged Madoff fraud, as strapped investors seek access to their cash amid increasingly hard times.

At least six suspected multimillion-dollar fraud cases have emerged this month alone, many of them alleged Ponzi schemes, in which investors are lured by promises of lofty returns but are actually paid off from new victims' funds.

On Tuesday, authorities arrested Arthur Nadel, the missing Florida hedge-fund adviser, who was accused by federal authorities of defrauding clients of millions of dollars. His arrest came as Spanish banking giant Banco Santander SA said it would offer thousands of its private-banking clients €1.38 billion ($1.82 billion) in compensation for losses arising from investments in Bernard L. Madoff's alleged $50 billion Ponzi scheme, the first financial company to do so.

Todd Foster, a lawyer for Mr. Nadel, says his client voluntarily surrendered to the authorities and is cooperating with them. He says "we aren't in a position to comment on the criminal charges now," adding that investment "losses don't necessarily equate with fraud."

In the latest case to emerge, Nicholas Cosmo, a Long Island, N.Y., investment-firm owner, surrendered to federal authorities Monday. Mr. Cosmo allegedly raised more than $370 million between 2006 and 2008 by promising investors 48% annual returns from funding commercial loans, according to a federal affidavit in support of his arrest.