Issa also accused the DHS's Office of General Counsel of instructing career staff not to search for documents he had requested.
"This directive is inconsistent with your pledge to identify and produce documents expeditiously, and it raises questions about the Department's commitment to the President's effort to create an 'unprecedented level of openness in government,'" Issa wrote.
Issa will start interviewing DHS officers, including Chief Privacy Office Mary Ellen Callahan, beginning on Feb. 9, according to the letter. Months after the Associated Press first reported about the delays, Callahan told Issa staffers last September that the government was not inappropriately meddling with FOIA requests. Issa reacted by putting the investigation on hold until he received unspecified new information last month, which caused him to rethink that decision. The Homeland Security Department's inspector's general's office also looked into the matter last year but has yet to make its audit public.
In a letter to Issa last week, Assistant Homeland Security Secretary Nelson Peacock said the department's "practices are fully consistent with the language and spirit of the Freedom of Information Act" and were vetted by agency attorneys.
Issa also demanded copies of the documents eventually released to the AP last year during its initial probe and for e-mails on the subject exchanged with the White House.
Republicans led by Issa are using the probe to target President Obama's pledge to provide greater government transparency. It is also an attempt to call the administration on the carpet for any attempts to stifle the flow of information to the media, an effort which will no doubt curry favor with journalists who rely on FOIA for investigative reporting.