For two weeks now, TPMDC has been tracking the mysteriously delayed nominations of John Holdren, named as the president's next chief science adviser, and Jane Lubchenco, slated to head the National Oceanic and Atmospheric Administration (NOAA).

A convincing, but still incomplete, trail of evidence points to Sen. David Vitter (R-LA), who won cheers from conservatives for sharply questioning Holdren during the nominees' confirmation hearing last month. But when I asked him directly, Vitter denied placing the hold, raising the question of whether his staff may have been raising objections on his behalf.**

After all, a similar situation occurred in the case of Sen. John Barrasso (R-WY), whose office stalled the confirmation of two other Obama environmental nominees in January. Barrasso aimed to use those nominees as leverage to meet with White House climate adviser Carol Browner, and he ended up getting what he was after. Could Vitter's staff be working a similar angle for him?

Strangely enough, Vitter's press office won't say. My multiple attempts to reach the senator's spokesman over the past few days have been unsuccessful. Why wouldn't the office simply confirm what Vitter told me himself, that he's not the source of the holdup?

We can rule out several other suspects in Holdren and Lubchenco's delay.

The first signs pointed to a simple and resolvable case, with the Washington Post reporting that Sen. Robert Menendez (D-NJ) had held up the nominees to draw attention to his concerns with a Cuba policy provision inserted in the 2009 spending bill.

But Menendez's involvement ended pretty quickly, as Senate Commerce Committee Chairman Jay Rockefeller (D-WV) confirmed that the nominees were actually facing multiple anonymous objections.

When I asked Rockefeller if the holders were Republican -- a reasonable assumption, given the alarms raised by conservatives over Holdren's candor on climate change -- the senator said he believed so, but couldn't be sure. Slowly but surely, the leading GOP suspects fell off the list:

Sen. James Inhofe (OK). He's long relished the role of climate-change denier, making him a perfect candidate to stand in Holdren's way in particular. But Inhofe's openness about his uber-conservatism on the environment -- he proudly stopped Al Gore's Live Earth concert from coming to the Capitol in 2007, for instance -- strongly suggests that he would admit to delaying the nominees if he were the culprit. And he told me directly that he's not.

Sen. John Barrasso (WY). He was similarly open in January when TPMDC reported on his objections to unanimously confirming two of the president's senior environmental nominees. Like Inhofe, he also directly denied holding up Holdren and Lubchenco.

Sen. Kay Bailey Hutchison (TX). She's the senior GOPer on the Commerce Committee, which has jurisdiction over the two nominees. But Hutchison doesn't have an exceptionally strong record of conservatism on climate issues, and she asked no confrontational questions during Holdren and Lubchenco's confirmation hearings. The senator also told me directly that she wasn't the source of the holdup and hadn't been aware of it.

That leaves only Vitter, the one Republican who did tee off on Holdren during the Commerce panel's hearing last month. As Climate Science Watch put it:

Some of Vitter's questions seemed to come straight out of a selective quotation effort that has been spun up by some elements within the global warming disinformation campaign in an effort to undermine Holdren's credibility.

National Review quoted a source in the room as admiring Vitter's confrontational posture towards the nominee: "[Vitter a]lso hammered him on the population control issues. It was beautiful!"

The informed speculation I've been picking up on the Hill pointed to a Commerce member, likely Vitter -- reporting by two other websites tracking Holdren and Lubchenco, Progressive Alaska and the Questionable Authority, also pointed to Vitter.

But with Vitter's ruling himself out, at least publicly ... the mystery continues.

**Late Update: The holds were cleared, and the nominees were approved, late on Thursday afternoon. The identities of the holders, as well as their political affiliations, unfortunately remain unknown.