Private mortgage bond investors are targeting $73 billion in securities issued by Wells Fargo (WFC) and Morgan Stanley (MS) in hopes of forcing the banks to buyback some of the underlying loans.

Lawfirm Gibbs & Bruns sent a notice to the two banks Wednesday identifying the bonds investors allege the two banks violated pooling and servicing agreements. Such notices often come after a settlement offer was rejected and could signal escalation ahead.

The firm alleged in a statement that "each of these failures has materially affected the rights of the certificateholders and constitutes an ongoing event of default in the mortgage servicer's performance under the relevant PSAs."

The notice covers $45 billion in bonds issued by Wells and $28 billion from Morgan Stanley.

Analysts at research firm Compass Point said more investors in private-label mortgage bonds would likely increase their claims as the six-year statute of limitations on 2006 deals draws near.

"Further, it seems logical that G&B, or others, are likely preparing additional actions such as these in preparation for lawsuits," Compass Point analysts said.

They estimate -- using figures from a separate settlement between investors and Bank of America (BAC) -- that Wells may be on the hook for $1.2 billion in claims and Morgan Stanley could face $800 million.

"We will review any communication we receive and respond appropriately," a Wells Fargo spokesperson said in a statement.

Morgan Stanley declined to comment.