New scrutiny of the Oklahoma City bombing points at a second man alongside Timothy McVeigh, which is bad news for the FBI's long-standing official account.

• A witness identified former Ku Klux Klan organizer Dennis Mahon as the "John Doe 2" driving the rental truck, with McVeigh in the passenger seat.
• Mahon had publicly bragged about hate crimes and said he would set off a nuclear device at the US Capitol if he could.
• He claimed he was at his parents' house in Illinois all that week, and the FBI set the witness aside because the sighting clashed with other accounts.
• The witness grew more certain after seeing Mahon on TV wearing the same style of sunglasses as the truck driver.
• Mahon himself has hinted that whoever carried out the attack would have used a decoy.

Outlook: Pressure will keep building on the FBI to explain why it dismissed witnesses who described a second bomber.