The couple, Tareq and Michaele Salahi, say they did not crash the party, insisting in a television appearance on Tuesday they had been invited guests -- a claim the White House has flatly denied.

In the latest twist to the saga, the publicist representing the Salahis said they have declined to testify at a hearing on Thursday by the House of Representatives Homeland Security Committee to investigate the incident.

Representative Bennie Thompson, the chairman of the committee, said later he was prepared to issue subpoenas to compel the couple to appear.

The publicist said that, through their attorneys, the Salahis had provided the committee with sworn statements and e-mail correspondence with a White House official.

The Salahis believe "there is nothing further that they can do to assist Congress in its inquiry regarding White House protocol and certain security procedures," the publicist's statement said.

The incident, which has an international dimension because the gate-crashers were within feet of Indian Prime Minister Manmohan Singh and they shook hands with Obama, has already drawn a rare public apology from the Secret Service.

The White House said it would ensure in the future that White House staff are physically stationed alongside Secret Service agents to screen guests at official events.

"After reviewing our actions, it is clear that the White House did not do everything we could have done to assist the United States Secret Service in ensuring that only invited guests enter the complex," White House Deputy Chief of Staff Jim Messina said in a memo released to the media.

"As the Secret Service said last week, agents failed to verify that these two individuals were invited guests before they entered the White House."

Daily White House media briefings have been swamped with questions about who was at fault for the error since it emerged last week, almost crowding out queries about Obama's strategy to send thousands more U.S. troops to Afghanistan.