While Williams' dismissal was legal and "not the result of special interest group or donor pressure," there were "concerns regarding the speed and handling" of the process, NPR said in a statement after the review. Independent directors worked with outside legal counsel to gather information.
Williams' NPR employment was terminated after controversial remarks he made about Muslims on Fox News -- which gave him a contract reported at $2 million for three years after the NPR dismissal.
NPR said on Thursday its board "has adopted recommendations and remedial measures designed to address issues that surfaced with the review."
Those "range from new internal procedures concerning personnel and on-air talent decisions" to "disciplinary action with respect to certain management employees" involved in the firing.
The board "expressed confidence in (chief executive officer) Vivian Schiller's leadership going forward," but also said it was concerned about her role in the firing and had "voted that she will not receive a 2010 bonus."
"Ellen Weiss, Senior Vice-President for News, has resigned," the statement also said without elaborating. Weiss was the NPR executive who informed Williams by phone that he was being fired.
In his remarks on Muslims on Fox's "O'Reilly Factor," where he was often a guest, Williams had said it made him nervous when he was flying and saw passengers in Muslim garb, though he also said all Muslims should not be blamed for extremists.
Muslim advocacy groups and others had attacked the comments and some called for NPR to fire Williams.
NPR said at the time that Williams' commentary was part of a pattern inconsistent with its editorial standards "and undermined his credibility as a news analyst with NPR."
The move drew fire from critics who, like Williams himself, argued it was his frequent appearances on conservative-leaning Fox that were the real issue and similar standards were not applied to NPR staff who made political comments on other media.
NPR's ombudsman had described the dismissal as poorly handled.