Acting Inspector General Charles Edwards said in a statement that field work was "beginning immediately," CNN reported Tuesday.
"We have maintained close contact with the USSS [U.S. Secret Service], coordinating with them as their internal investigation continued," Edwards' statement issued Monday said. "Last week, with the initial stages of the USSS internal investigation nearing completion, we notified the USSS of our intent to conduct a comprehensive review of this matter. As we notified the USSS last week, OIG's [Office of Inspector General] field work is beginning immediately."
Edwards' announcement came as Secret Service Director Mark Sullivan faces two deadlines Tuesday to answer questions about the scandal.
Sullivan had to answer 50 questions submitted by Homeland Security Chairman Peter King, R-N.Y., while House Oversight Committee Chairman Darrell Issa, R-Calif., and Rep. Elijah Cummings, D-Md., the panel's ranking Democrat, said they have questions of their own, including a request for a timeline of events in Cartagena, Colombia, before President Obama arrived for a summit.
Issa and Cummings also asked Defense Secretary Leon Panetta for details of the military investigation by May 8. In their letter to Panetta, the congressmen said security personnel displayed an "alarming lack" of "character" and "judgment."
The incident last month before Obama's trip to the Summit of the Americas in Colombia involved Secret Service and U.S. military members who allegedly hired prostitutes. Twelve Secret Service personnel and 12 members of the military have been tied to the scandal.
Nine Secret Service members resigned or were being forced out, and three others were cleared of serious misconduct. A separate military investigation hasn't announced any measures taken.