WASHINGTON – Three more Secret Service officers resigned Friday in the expanding prostitution scandal that has brought scorching criticism of agents’ behavior in Colombia just before President Obama’s visit for a summit meeting last week. Agency Director Mark Sullivan personally briefed Obama in the Oval Office late Friday.

The Secret Service announced the new resignations, bringing to six the number of agency officers who have lost their jobs so far because of events at their hotel in Cartagena.

Also late Friday, Sen. Chuck Grassley, R-Iowa, urged a broader investigation, including whether White House advance staff and communications personnel may have shared rooms with the Secret Service at the Caribe or other hotels in Cartagena. In a letter to Sullivan and the inspector general at the Department of Homeland Security, Grassley asked whether hotel records for the White House staffers had been tracked down.

An additional Secret Service employee was implicated Friday, a government official said, commenting only on condition of anonymity concerning the continuing investigation. That brings the number to 12. One has been cleared of serious misconduct but still faces administrative action, an official said.

Obama’s spokesman has assailed Republican criticism that has attempted to blame a lack of presidential leadership for the scandal and has said Obama would be angry if allegations published so far proved to be true. Friday’s was Obama’s first personal briefing by Sullivan on the subject, officials said.

Involvement by 11 Secret Service employees had been noted earlier. The 12th has been placed on administrative leave.

The scandal also involves at least 11 military members who were working on security before Obama arrived in Cartagena for the Summit of the Americas. The Pentagon said Friday that the 11th military person, a member of the Army, was implicated.

The incident in Colombia involved at least some Secret Service personnel bringing prostitutes to their hotel rooms. News of the incident, which involves at least 20 Colombian women, broke a week ago after a fight over payment between a prostitute and a Secret Service agent spilled into the hotel hallway.

A 24-year-old Colombian prostitute told The New York Times that the agent agreed to pay her $800 for a night of sex but the next morning offered her only $30. She eventually left the hotel, she told the newspaper, after she was paid $225.

Two Secret Service supervisors and another employee were forced out of the agency earlier in the week. All of the agents being investigated have had their top-secret clearances revoked.

Meanwhile, the lawyer for two Secret Service supervisors said that Obama’s safety was never at risk, and he criticized leaks of internal government investigations in the case, signaling a possible strategy for an upcoming legal defense.