By David Morgan

WASHINGTON, Oct 22 (Reuters) - Congressional Republicans on Tuesday announced a new investigation into the troubled rollout of President Barack Obama's health care reforms, aimed at learning what role the White House may have played in decisions about the design and development of problem-plagued website Healthcare.gov.

In a letter to two top White House technology officers, Republicans on the House of Representatives Oversight and Government Reform Committee said their investigation already points to significant White House involvement in discussions between the Centers for Medicare and Medicaid Services (CMS) and contractor CGI Federal.

CGI officials have also told committee staff that the widely criticized design feature requiring visitors to create accounts before shopping for insurance was implemented in late August or early September, barely a month before the Oct. 1 start of open enrollment.

The requirement is said to have led to a traffic bottleneck that worsened underlying flaws in a system intended to serve millions of Americans under the 2010 Affordable Care Act, also known as Obamacare. The technology problems have frustrated attempts by many to sign on and allowed only a trickle of enrollments.

The probe, the second House Republican investigation into Obamacare, is the latest example of efforts by the party to advance their opposition to the law after failing to derail it during a 16-day government shutdown in October. For Republicans, the healthcare law is an unwarranted expansion of the federal government.

Obama said on Monday he was frustrated by the website's problems and that the administration had called in leading technology experts to help to fix it. A prolonged delay in getting Healthcare.gov to work could jeopardize the White House's effort to sign up as many as 7 million people in 2014, the first full year it takes effect.

The White House and the Department of Health and Human Services, which oversees CMS, had no immediate comment on the Oct. 21 letter addressed to U.S. Chief Information Officer Steve VanRoekel and U.S. Chief Technology Officer Todd Park.

"We are concerned that the administration required contractors to change course late in the implementation process to conceal Obamacare's effect on increasing health insurance premiums," said the letter authored by panel chairman Darrell Issa and four Republican subcommittee chairmen.

Under Issa's leadership, the oversight committee has pursued the Obama administration on one matter or another since Republicans took control of the U.S. House in the 2010 elections. As chairman, or earlier as its senior Republican when Democrats controlled Congress, Issa established a record of leveling accusations against the White House and demanding reams of documents that then become administrative minefields.

They have included inquiries into aspects of the financial crisis, the September 2012 attacks on the U.S. diplomatic mission in Benghazi, Libya, and allegations this year that the Internal Revenue Service targeted applicants for tax exempt status because of their political beliefs.

The House Energy and Commerce Committee has started its own investigation and is scheduled to question U.S. Health and Human Services Secretary Kathleen Sebelius and several contractors at separate hearings within the next eight days.

Issa's committee is asking VanRoekel and Park to provide all documents and communications that describe the federal system's architecture and design, CMS' role as system integrator, problems relayed to the White House and the decision to require account creation as a prerequisite to seeing insurance plans.