Here's the explanation from Dobbs:
The first allegations of inappropriate activity involving computers owned by Mayor Becker were presented to D.C. Tom Christensen and Chief Wahlen on or about December 23, 2008. During these initial meetings between the Chief and Mike Ferderer, Ferderer had mentioned that Paul Burdick may have also seen some inappropriate materials in one Mayor Becker's computers some time ago when Burdick worked on Becker's computer.
Chief Wahlen saw Burdick off duty on Sunday, 1-11-09. Chief Wahlen started a conversation asking about computer repairs performed in the past. Burdick eventually offered up that he had performed work on Mayor Becker's personal computer in 2007. Burdick went on to say that he observed some inappropriate sexual material at that time. Burdick stated that when he gave the repaired computer back to Becker, he advised Becker that he should remove that type of material from his computer. Burdick also stated that he gave Becker a bill for services rendered. Burdick then advised Chief Wahlen that he had made a copy of the material on Becker's hard drive. Chief Wahlen then obtained that copy and forwarded it to DCI.
So that clears up any question of when police found out about the 2007 hard drive, which contained the picture of the naked 10-year-old. It doesn't explain why Burdick had a copy of the contents of Becker's personal computer (is there a way to prove the contents on the CD came from Becker's computer?), and why he didn't turn it over to police sooner. Then again, if you're a city employee and you come across potentially damaging information on the mayor, maybe the best thing to do is cover your tracks and keep your mouth shut.
One last note: This seems to clear Wahlen of any sort of cover up. Becker may have appointed him police chief, but it looks like Wahlen personally took the lead on investigating the allegations and found strong evidence that, eventually, led to the mayor's arrest.
One last, last note: It's interesting that Burdick told Becker to remove the pornography from his computer. That's a slick way of letting the mayor know that he knows about the kiddie porn. It also means that Becker has known since 2007 that a city employee had the whamee on him - and could play it at any time.