One could try Googling "Sekhemkhet pyramid", and "Zakaria Goneim" for the background. There are some good webpages on this Old Kingdom site and the excavations (a fascinating story in themselves). The 19th dynasty mat burial labelled "Ka Nefer Nefer" above the rubble of the demolished pyramid complex is mentioned only in passing in some of them.
Googling "St Louis mummy mask" would bring up more about the subsequent history of this item once it had been divorced from its true provenance. This should throw up a couple of nice newspaper articles, including some from Egypt from which one can reconstruct the details of the controversy as it developed and then reached stalemate due to US foot-dragging.
Then there are blogs, forums and websites. I recommend SAFE and SAFE Corner, David Gill's "Looting matters", Tom Cremers' "Museum Security Network" in its several incarnations and a few others (look in the list of links in my sidebar). They all have several posts on this matter and related issues. There is nothing much of any is use on any of the collectors' forums, blogs and so on.
Two online texts in particular are very helpful. The first is a well-written and lengthy piece of investigative journalism which names names and fills in a lot of detail. This is the text of Malcolm Gay of 15 Feb. 2006 for the Riverfront Times, entitled "Out of Egypt: From a long-buried pyramid to the Saint Louis Art Museum: The mysterious voyage of the Ka-Nefer-Nefer mask"The second text that is well worth reading (and not just for the light it sheds on this case) is a post graduate thesis available online. Laura Elizabeth Young, "A Framework for Resolution of Claims for Cultural Property" (MSc in Arts Management, University of Oregon, December 2007). Chapter 3 is a fairly detailed account of the St Louis case, and provides a few additional details about what documentation SLAM posessed at the time of purchase and what joined the files after which clarifies the sequence of events better than the account given earlier by Gay.
The Aboutaams should need no introduction (if they do, Google them and "Pheonix Ancient Art" too)
In particular pay attention to the part played in creating a provenance by a Swiss man named Charly Mathez, who in 1997 attested that he'd seen the Ka-Nefer-Nefer at a Brussels gallery 45 years earlier, but who has - it turns out - fallible memory when asked for details. In 1952 this object was, acording to Egyptian documentation, still in the Sakkara storerooms.
Then there is the Croatian connection. The mysterious 'Kaloterna (Kaliterna?) collection. A half million dollar Egyptian antiquity is stated by the Aboutaams/SLAM to have gone from a Brussels gallery to Croatia in Communist Yugoslavia in the early 1960s. Then it came into the hands of an 'anonymous' collector in Switzerland who gives a provenance of the "early sixties". This Gay discovered was a 'Zuzi Jelinek', who turns out to be a real person, Suzanna Jelinek of Zagreb in Croatia... who it seems was the Aboutaam's landlady in Geneva. A letter from her is the main piece of evidence of where the object was before the Aboutaams obtained it. Then there is the role of Sidney Goldstein, the museum's antiquities curator who initiated and oversaw the mask's purchase in 1997, Brent Benjamin only became director a year later. If Laura Young is right about what joined the SLAM files when, then it would seem they were less than open with Gay and others back in 2006 about what "due diligence" they had in fact applied at the time.