The Secret Service's efforts to improve the conduct of agents working abroad is not enough for GOP Sen. Chuck Grassley, who continues to call for independent investigations and be one of the party's most persistent members on the issue.

The Secret Service issued a 10-point conduct code late Friday, about two weeks after at least one agent allegedly brought a prostitute back to his hotel room in Cartagena, Colombia, as a team of agents prepared for President Obama's arrival for an April 14-15 summit in the country.

"The new conduct rules are necessary to prevent more shenanigans from happening in the future," said Grassley, R-Iowa and ranking member of the Senate Judiciary Committee. "A sheet of paper with new rules doesn't negate the previous actions. … It remains necessary to hold the agency and the agents accountable, following a complete and independent investigation."

The new rules include chaperones on some trips, no excessive drinking, no foreigner nationals in hotel rooms and no visits to disreputable establishments.

Grassley, whose committee has oversight of the Secret Service, has pressed for answers since the scandal unfolded and perhaps widened. He has been particularly skeptical about whether similar incidents occurred in years past and if White House aides also traveling ahead of the president were involved in the scandal. It started when the prostitute became upset in the hotel hallway about her fee and allegedly included dozens of other prostitutes.

Twelve agents and 12 military personnel have been identified in the incident. Eight of the agents have been forced out, one had his security clearance revoked and the remaining three have been cleared.

CNN and other news agencies identified the agent at the center of the scandal late Friday.

Grassley also said Friday the White House failed to respond to a letter sent by the Democrat-led Senate committee giving administration officials 48 hours to respond to 14 questions about whether staffers were involved in the scandal. Grassley said he also sent several emails to the White House as reminders of the impending deadline.

The White House counsel Kathryn Ruemmler did her own investigation absolving the staffers involvement. But Grassley thinks the investigation is not credible, given its lack of independence.

Secret Service and other federal officials have said they have no solid evidence to support news reports about similar incidents in Buenos Aires, Moscow and San Salvador dating as far back as 2000. However, Grassley remains unconvinced.

"This culture can't persist, not only for the reputation of the United States but also for the protection of the president," he said Friday on CBS's "This Morning."

Grassley's remark came two days after Homeland Security Secretary Janet Napolitano told committee members an internal Secret Service investigation has thus far gone back two-and-a-half years and found no similar complaints.

"Don't you think we ought to make sure before two-and-half years it wasn't a problem," Grassley asked Napalitano.

Napalitano also said the investigation was being conducted under the guidance of  her department's inspector general.

Grassley said he had confidence the inspector general could do the job but was "bothered" by Napolitano saying there was a "memorandum of understanding" among the IG, the Secret Service and other divisions with the department.

 "That's not the independence expected of an inspector general," Grassley said.Â