One of the people Leahy wanted confirmed

to get his 'cooperation' on investigating Gunwalker?
To recap, James Cole is no stranger to serious misconduct occurring during his alleged “watch.” Cole was the “Independent Consultant” stationed at AIG by the Securities and Exchange Commission (SEC) before, during and after the financial meltdown of 2008. Despite his oversight responsibilities, Cole looked the other way as the AIG Financial Products Division in London ran the corporation off the rails, ultimately driving the international economy off a cliff. The cost to US taxpayers of the AIG bailout was more than $150 billion.

Fast forward to January 2011. After President Obama installed Cole at DOJ as the Acting Deputy Attorney General through a recess appointment, Senate Judiciary Committee Ranking Member Charles Grassley (R-Iowa) and House Oversight & Government Reform Committee Chair Darrell Issa (R-California) began asking Cole’s office for information about F&F. On January 27, Grassley wrote to ATF Acting Director Kenneth Melson, requesting information. Four days later, Grassley again wrote to Melson about reports that whistleblowers on F&F were being silenced, stating “Rather than focusing on retaliating against whistleblowers, the ATF’s sole focus should be on finding and disclosing the truth as soon as possible."
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Grassley, of course, has crossed swords with Cole before. Through 2010 and into 2011, Grassley blocked Cole’s confirmation in part because Cole had failed in his oversight at AIG. In June, however, Grassley withdrew his objection to Cole in exchange for information from the DOJ about F&F (Just for now, let’s ignore the dubious practice of forcing Congress to exchange votes for information from executive agencies about wrongdoing). Cole was then confirmed on June 27, and shortly after the vote, DOJ turned over documents about F&F to the Senate.

In the documents, Grassley discovered that Cole had been informed about F&F in March by Melson and had participated in concealing the details of the operation from Congressional investigators.
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Let's recap:

Cole ignored or failed to recognize financial risk at AIG for years (while collecting a hefty fee).
Grassley blocked his confirmation as Deputy Attorney General.
Using a recess appointment, President Obama appointed Cole to Deputy Attorney General in December of last year, and Cole took up his position at DOJ.
Cole’s office at DOJ withheld information from Congress about F&F at ATF, a program that allowed thousands of firearms bought in the US to fall into the hands of criminals in Mexico.
In a deal with Democrats, Grassley agreed to allow a confirmation vote on Cole in exchange for access to information from DOJ about F&F.
Upon receiving that information, Grassley learned that a DOJ official partially responsible for withholding F&F details from Congress was … Cole himself.
And if you don't believe Leahy knew, well, I've got some bridges I'd like to sell you.