A Treasury official's testimony before Congress has shaken investors' confidence in Fannie Mae and Freddie Mac - and increased the secondary marketing giants' borrowing costs.

In the wake of testimony Wednesday by Gary Gensler, undersecretary for domestic markets, Fannie's stock Thursday fell as much as 6%, to $58, before rebounding; it ended the day at $62.50, up 1.03%. Freddie's stock fell as much as 8%, to $42.75, before rebounding to $47, up 1.21%.

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