It amounted to only two paragraphs at the end of a 475-page bill to create the Department of Homeland Security. But the brief provision -- designed to shield vaccine makers such as Eli Lilly and Co. from lawsuits seeking billions of dollars for families of autistic children -- has generated a whirlwind of controversy and a mystery as to its origin.

The paragraphs appeared just days before the House was to vote on the legislation. House Republicans rammed the bill through during Congress's "lame duck session" and sent it to the Senate, where Democrats, demoralized by the Nov. 5 election results, could not to stop it.

And so, with little debate, Congress granted broad legal protection to the makers of Thimerosal, a preservative in childhood vaccines that has been circumstantially linked to rising rates of autism and pediatric developmental problems. It seemed a lobbying coup for Lilly and its allies. Yet, strange to say in Washington, no one seems to want to take credit.

Pharmaceutical lobbyists, Eli Lilly representatives and lawmakers with the most knowledge of the Thimerosal issue have denied any role in the provision's last-minute appearance. Now, White House budget director Mitchell E. Daniels Jr., a former Lilly executive, is the latest person to formally deny a part. He did so in a sharply worded response to an accusatory letter by Rep. Henry A. Waxman (D-Calif.).

Daniels said the provision was not approved or developed by the White House Office of Management and Budget, adding:

"I also want to make clear that I personally had no involvement whatsoever with these provisions. I spoke to no one about these provisions, either inside the administration or outside the administration. . . . I did not have any communications with anyone from Eli Lilly regarding the issue. Indeed, I had not even heard of Thimerosal until I received your letter, which is not surprising because Eli Lilly stopped making Thimerosal a decade before I began working there and the lawsuits appear to have been filed after I left."

Since the provision's appearance, some Democrats and trial lawyers have charged that it represented a timely payback for the pharmaceutical industry's financial support in the midterm elections. "President Bush and conservative Republicans are going to give the pharmaceutical companies whatever they ask for," said Michael Williams, an Oregon lawyer who represents several families of autistic children and believes billions of dollars could be at stake.

Under the provision, a raft of Thimerosal lawsuits will be redirected from state courts to the federal Vaccine Injury Compensation Program, which caps damages and sharply limits who can file suits against vaccine makers. Proponents say the provision merely closes a loophole, which had been exploited by trial lawyers claiming that Thimerosal was a vaccine "contaminant" not subject to existing legal regulations. If action was not taken, advocates say, the lawsuits could have driven vaccine makers out of business.

The provision was drafted more than a year ago by Sen. Bill Frist (R-Tenn.) as part of a broader bill to revise the vaccine program. That bill, which Frist had hoped to begin action on next year, includes measures favoring plaintiffs as well as manufacturers. It would raise the cap on damages, extend the statute of limitations for filing suit and allow the parents of autistic children to sue on their behalf.

An aide to retiring House Majority Leader Richard K. Armey (R-Tex.) said Armey's staff put the Thimerosal provision in with no prodding from the pharmaceutical industry or the White House.

But several corporate lobbyists said that is not credible. Whoever was responsible had to have detailed knowledge of the legal issues, had to know Frist had drafted the larger bill, and had to understand exactly which provision applied to Thimerosal because the brand name does not appear in the text. Two sources said an official at the Department of Health and Human Services gave the final approval, a statement that HHS spokesman Bill Pierce adamantly denied.

What is clear is that as recently as two months ago, lobbyists for Lilly and other drug makers were on Capitol Hill trying to get the entire Frist vaccine bill inserted into the homeland security legislation. But, the lobbyists said, they were as surprised as anyone when the two-paragraph item was included.

One senior Republican Senate aide said a member of Frist's staff received a call just days before the House passed the homeland security bill, saying he had heard a rumor that the Thimerosal provision was included. The Frist aide said the lobbyist was confusing that provision with another measure to protect makers of smallpox vaccines. The next day, the aide said, Frist's staff found the Thimerosal provision in the bill as they scanned it in the Senate cloakroom.

"We don't know how it became part of the House bill," said Rob Smith, a Lilly spokesman. "We didn't know it was part of the bill, and it was a surprise to us."

The provision could be the lobbying coup of the 107th Congress. A series of ongoing academic studies should be able to conclude within the next three years whether Thimerosal, a mercury-based additive, can be scientifically linked to an upswing in autism, Williams said. Absent the two-paragraph provision, such a conclusion could open the legal floodgates.

Yet corporate lobbyists who might be expected to crow about saving their clients potentially billions of dollars have stayed mum. That may be in part because the deed was done rather clumsily, one lobbyist said. The provision was not even hidden. Instead, it was simply tacked on at the end of the bill. That has brought down a wave of unwanted publicity on vaccine makers, especially Lilly, the inventor of Thimerosal.

"They didn't even make an effort to be clever about it," the lobbyist said.

Budget director Mitchell E. Daniels Jr., a former Lilly executive, denies having any role in language involving Lilly product Thimerosal.