The incident involving U.S. Secret Service agents and members of the U.S. military allegedly hiring prostitutes before President Barack Obama visited Colombia was "almost certainly" not isolated, according to Sen. Susan Collins of Maine.

Collins' comments come in an opening statement she plans to give Wednesday morning at a hearing of the Senate Homeland Security Committee. She is the ranking Republican on the panel.

The director of the Secret Service, Mark Sullivan, is set to testify at the hearing. The committee is investigating the use of prostitutes by Secret Service agents in Colombia last month before Obama's visit - one of four congressional committees looking into the incident.