Investment bank Piper Jaffray has agreed to buy Seattle-Northwest Securities, a well-established regional underwriter of municipal bonds, for about $21 million in cash, the firms said Thursday.

Minneapolis-based Piper sees the acquisition as a step toward expanding its public finance business nationally, the firm said in a statement.

Seattle-Northwest ranked first among underwriters of general obligation bonds in Washington, Oregon and Idaho in 2011, with 51 issues totaling $739 million, according to its website. Piper Jaffray ranked fourth with eight issues worth $146 million.

The local firm, owned by an employee stock ownership plan (ESOP), had average revenues of $17 million over the past three years, according to a Piper Jaffray presentation.

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