An audit by San Francisco county officials of about 400 recentforeclosures there determined that almost all involved either legal violations or suspicious documentation, according to a report released Wednesday.

Anecdotal evidence indicating foreclosure abuse has been plentiful since the mortgage boom turned to bust in 2008. But the detailed and comprehensive nature of the San Francisco findings suggest how pervasive foreclosure irregularities may be across the nation.

The improprieties range from the basic — a failure to warn borrowers that they were in default on their loans as required by law — to the arcane. For example, transfers of many loans in the foreclosure files were made by entities that had no right to assign them and institutions took back properties in auctions even though they had not proved ownership.

Commissioned by Phil Ting, the San Francisco assessor-recorder, the report examined files of properties subject to foreclosure sales in the county from January 2009 to November 2011. About 84 percent of the files contained what appear to be clear violations of law, it said, and fully two-thirds had at least four violations or irregularities.

Kathleen Engel, a professor at Suffolk University Law School in Boston said: “If there were any lingering doubts about whether the problems with loan documents in foreclosures were isolated, this study puts the question to rest.”

The report comes just days after the $26 billion settlement over foreclosure improprieties between five major banks and 49 state attorneys general, including California’s. Among other things, that settlement requires participating banks to reduce mortgage amounts outstanding on a wide array of loans and provide $1.5 billion in reparations for borrowers who were improperly removed from their homes.