Spokesmen for Standard & Poor’s and the S.E.C. declined to comment.

Companies have been quicker to disclose that they received Wells notices since last year, when Goldman was criticized for not having disclosed the one it received related to the S.E.C.’s mortgage security investigation.

Delphinus was just one of many ill-fated mortgage securities called collateralized debt obligations, or C.D.O.’s, that represented bundles of mortgage bonds which were themselves bundles of home loans. The securities were supposed to be diversified so that if some homeowners stopped paying their bills, others loans inside the securities would be unlikely to default at the same time. But many of those securities turned out to be full of poorly underwritten mortgages that defaulted at the same time.

In the past, when the S.E.C. has looked into mortgage cases, it has focused its investigations on one deal per company. Its case against Goldman, for instance, was centered on only one deal called Abacus, even though there were nearly 20 other similar deals at Goldman. It was unclear if Delphinus, which was arranged by a unit of Mizuho Financial Group, would be the only one the S.E.C. pursued to see if S.& P. violated federal securities laws.

There was so much demand for the Delphinus 2007-1 deal in July 2007 that Mizuho Securities increased its size from $1.2 billion to $1.6 billion, according to a Mizuho news release. The bank’s head of structured credit for the Americas said in that release that investors wanted “better quality collateral.” A spokesman for Mizuho declined to comment.

Standard & Poor’s and other rating agencies were not generally the architects of deals like Delphinus, but the AAA ratings they placed on parts of those deals were critical to the banks’ abilities to sell them to investors. S.& P. and other agencies made record profits placing ratings on mortgage securities like Delphinus, but they did not provide any sort of promises to investors that their ratings were accurate.

If investigators at the S.E.C. or the Justice Department find that analysts at S.& P. intentionally gave inaccurate ratings, it could be a violation of the law.

The Senate subcommittee report in April said that in 2006, S.& P. made $561 million in revenue in its structured finance group, where mortgage bonds were rated, and that the firm charged from $30,000 to $750,000 to rate each deal. In the three years before the crisis, S.& P. rated 5,500 mortgage bonds and 835 C.D.O.’s, many of them AAA, despite being aware of increasing risks, the report said.