(CNN) -- The ranking Republican on the House Homeland Security Committee accused the White House of "stonewalling" Thursday by blocking its top social planner from testifying at a hearing on a security breach.
New York Rep. Peter King was angered by the administration's refusal to let Social Secretary Desiree Rogers speak at a hearing examining how a Virginia couple was able to enter the White House for a state dinner last week, even though they were not on the guest list.
Rogers' office planned the dinner.
Secret Service Director Mark Sullivan testified Thursday that there was a lone agent at the initial checkpoint where Tareq and Michaele Salahi entered the White House grounds. He acknowledged that if someone from the social secretary's office had been there, the couple may have been stopped.
At many past events, there has been a White House representative present to help identify guests, Sullivan said. He added that it was decided Wednesday that that will be done in the future.
King wasn't mollified.
He said the only way to determine what happened in the exchange between the Salahis and the gate guard November 24 was to have someone from the White House fill committee members in.
"We can't do it unless we have someone from the White House having the guts to come down here and testify, instead of hiding behind a phony claim of separation of powers," King said.
Addressing committee Chairman Rep. Bennie Thompson, he noted, "This was a bipartisan request, Mr. Chairman. ... In this instance, they are stonewalling."
Thompson said he didn't see a reason for Rogers to testify, because the hearing was focusing on security issues.
Sullivan acknowledged that "appropriate procedures were not followed."
He said a preliminary investigation into the incident has been completed, and the guards involved have been placed on administrative leave with pay.
He said that once the extent of culpability is determined, "appropriate action" will be taken.
"I regret ... that established protocols and procedure were not followed," Sullivan said, calling the breach that began at the White House entry checkpoint "unacceptable and indefensible."
The Salahis contend that they were told they could attend the program to honor India's prime minister, but the White House says they were not invited and were not on the official guest list for the exclusive affair, President Obama's first state dinner.
At one point during Thursday's hearing, a staffer raised a poster-sized photo of Michaele Salahi posing with Vice President Joe Biden at the event.
The Salahis were asked to appear before the committee Thursday but didn't.
"Maybe they didn't show because they were on the guest list," quipped Rep. Ann Kirkpatrick, D-Arizona.
After Sullivan's testimony, Thompson announced that he had asked staff to prepare subpoenas for the couple in an attempt to make them testify.
If they continue to rebuff the oversight request, they could be found in contempt of Congress, the Mississippi Democrat said.
He said the committee would discuss the subpoenas next week.
"There were undeniable planning and execution failures of the entire Secret Service apparatus," he said Thursday.
The ranking Republican on the committee, Rep. Peter King of New York, said he supports Thompson's intent to subpoena the Salahis, but he also wanted a subpoena issued for Rogers.
According to Sullivan, the Secret Service guard at the initial checkpoint should have called someone at the White House when he noticed the couple was not on the guest list. Instead, he waved them through. The guest list is prepared by the White House and vetted ahead of time by the Secret Service, Sullivan said.
Sullivan said there were three vehicle checkpoints and two pedestrian ones, and a guest list was at each, meaning the Salahis had to move through several layers of security checks.
"Were they on any of those lists?" Thompson asked.
"They were not," Sullivan replied.
He said those attending a planning meeting before the dinner decided that a single guard at the entry checkpoint would be sufficient.
However, he said it was understood that if that person had a question involving the guest list, the guard would summon someone from the White House staff to settle the issue.
"Why did someone from the Secret Service decide that made sense?" asked Rep. Dan Lungren, R-California, referring to the single guard.
When Sullivan said he didn't know who participated in the planning session, Rep. Mike Rogers, R-Alabama, demanded a list of everyone who attended.
King also wanted to know why the person who oversaw that decision wasn't at the hearing.
Sullivan said he has made or is trying to make improvements at the Secret Service by improving training, retention and recruitment and adding more managerial oversight at White House events.
He said a "resolution help desk" staffed by someone in the uniform division and a senior White House staffer was being set up. The agency has something comparable now, he said, but it is a mobile unit. The new one would be stationary.