WASHINGTON - Credit agencies are monitoring their ratings on transit agency leaseback transactions at a time when they are at growing risk of having to cough up billions of dollars as lease agreements with private investors go into default because of insurers' rating downgrades.

Standard & Poor's warned Wednesday that transit agencies face "credit factors" that necessitate close monitoring, within hours of Moody's Investors Service downgrading insurer Ambac Assurance Corp. to Baa1 with a developing outlook from Aa3 on review for downgrade.

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