Yet more of Washington is balking at the Internal Revenue Service, with the news that the agency had singled out conservative groups applying for tax-exempt status for tougher scrutiny. On what NBC's First Read team called his worst day politically since his painful presidential debate, President Barack Obama vowed Monday to get to the bottom of it, but five big questions remain. How did it start? "I just don’t buy that this was a couple of rogue IRS employees," Sen. Susan Collins (R-Maine) said. Who knew what when? The IRS said over the weekend that senior executives weren't aware of the targeting — but on Monday the IRS said its current acting commissioner learned of it in May 2012. Will anyone be fired? That's what Sen. Marco Rubio (R-Fla.) wants. And damage control? The agency is sure to face fierce grilling in Congress from members of both parties. How can this be prevented in the future? Some in Congress want to toughen the penalties for IRS agents.