We may have found out why U.S. Attorney Carol Lam was such a "problem" for the administration. Evidence points directly at Vice President Cheney as the one with the problem.
The day the news broke that a federal corruption probe in Southern California was spreading to Republicans, Rep. Jerry Lewis, the chief of staff to Attorney General Alberto R. Gonzales fired off an e-mail to the White House about the federal prosecutor who had begun the investigation.
"The real problem we have right now is Carol Lam," D. Kyle Sampson told White House Deputy Counsel William Kelley on May 11. "That leads me to conclude that we should have someone ready to be nominated 11/18, the day her 4-year term expires."
Los Angeles Times
This cryptic message has led some to charge the administration with obstruction of justice on the assumption that it was the pending investigation of Rep. Lewis that was the "problem". However, piecing together other pieces of the puzzle may indicate that was just the straw that broke the camel’s back.
To understand, we need to go back to the case of Rep. Randy "Duke" Cunningham who pled guilty to federal corruption charges involving government contractors who funneled large sums to the Congressman. One of those contractors was an outfit named MZM which, among other things, got as its first contract since the company was formed for $140,000 to furnish the office of (get ready for the kicker) Cheney.
Later, MZM would go on to get government contracts worth many millions of dollars and, in return, pay for a large portion of Duke’s lavish lifestyle. It turns out that the money Cheney’s office paid to MZM for their first contract is exactly the amount of money MZM in turn paid for a yacht that Cunningham lived on while in DC and used for his parties. 
It has been stated that Lam continued to follow up on the Cunningham case. Days before her firing, Lam was investigating Brent R. Wilkes, a defense contractor tied to Cunningham and Lewis. Could Cheney have been another target of those investigations?
Ms. Lam has declined to speak about the pending investigations that seem to be a problem to this administration, which is proper for her to do. That is why it is imperative that the House and Senate go through with this investigation no matter where it leads.
The explanations have changed over and over and now, as it is clear from the limited emails provided by the Department of Justice, it is only made worse. 
The current justifications are twofold: (1) the president has the right to fire a U.S. attorney at will and (2) they were fired for performance reasons.
I want to focus on Carol Lam’s case because it seems to expose the fraud being put out most clearly. Lam’s boss, Assistant Atty. Gen. William E. Moschella, praised her for doubling the number of prosecutions of immigration violations, undercutting one of the justifications used by the Administration. 
Lam has routinely received praise from DOJ officials and other U.S. attorneys, especially for her groundbreaking corruption cases. Not until the firing discussions had targeted her as one of the eight did claims of dissatisfaction over her immigration cases arise.
As for the "serving at the pleasure of the president" line of argument, it is absolute horse pucky and completely irrelevant. Ask Richard Nixon. Yes, they serve at the pleasure of a president, but not if the removal comes as an attempt to obstruct an ongoing or potential investigation. That is a crime, the kind that gets presidents impeached.
Both the Senate and House committees have said they will issue subpoenas of key players in the firing debacle. The president has said he will not comply based upon executive privilege. There are surprisingly few cases that have actually ruled on this issue, most having been settled by a compromise agreement between the president and Congress. 
But those that exist, most notably involving pres. Nixon, make clear that if a reasonable suspicion of a crime is involved, executive privilege must defer the subpoena power of Congress.
One way to force the issue is to transform these from committee investigations into a formal impeachment proceeding. The president cannot hide behind privilege in that setting. It is time for Congress to strap on a set of steel ones and get to the bottom of this and the many other "high crimes and misdemeanors" of this administration.