Allen, 23, of Conway, Ark., won by an undisclosed margin over Lambert, 27, of San Diego, a powerful singer who'd been cast by the show's judges as the front-runner. Host Ryan Seacrest said nearly 100 million votes were sent by phone or text message after last Tuesday's final performance, but did not mention how those votes were divided.
While anyone in the United States could phone in votes, only AT&T customers were allowed to text.
About 20 phones were brought to two separate parties in Arkansas, and two of those phones were capable of sending multiple votes by so-called "power texting," a person familiar with the gatherings said. A second person, with knowledge of the voting results, said Allen won by a large enough margin that a handful of voters could not have swayed the outcome.
Both spoke on condition of anonymity because they were not authorized to comment on the voting process or the Arkansas parties.
"Idol" prohibits "power voting" by text or phone, and has internal safeguards against the practice.
In a joint statement, Fox and producer FremantleMedia said the results were "fair, accurate and verified" by an independent, third-party monitor to protect the integrity of the voting process. "In no way did any individuals unfairly influence the outcome of the competition."
"Kris Allen is, without a doubt, the American Idol," Fox and FremantleMedia said.
According to AT&T, "a few local AT&T employees" were invited to attend local viewing parties in Arkansas for the "American Idol" finale, parties the company said were similar to other, "countless" ones held in "homes, bars and other public places" nationwide.
"Caught up in the enthusiasm of rooting for their hometown contestant, they brought a small number of demo phones with them and provided texting tutorials to those who were interested," AT&T said in its statement.
Dallas-based AT&T, the country's largest telecommunications company, is one of the show's main sponsors, though it did not sponsor the parties in question.
"Going forward, we will make sure our employees understand our sponsorship celebrates the competition, not individual contestants. That said, it's quite a leap to suggest that a few individuals could have impacted the final results," the company said.
An AT&T spokesman declined to address how many employees were invited or how many people attended the Arkansas parties.
One of the parties attended by AT&T was at the University of Central Arkansas' Estes Stadium, with about 2,000 people in attendance, the Arkansas Democrat-Gazette reported last week. The paper said display phones and company representatives were on hand to teach multiple "power texting" there, with phones and representatives also available at a watch party at the Peabody Little Rock hotel.
Sonja Keith, part of the "Kris Allen Kick Awesome Task Force" that helped organize watch parties, said she contacted AT&T about coming to the party at the stadium in Conway because she'd heard some people were having difficulty getting through on the toll-free number.
"A suggestion was made that we needed to get in touch with the phone carriers to see if there was a problem - if there was something that they could do to ensure that our calls were getting through."
Keith said the business was one of many that participated at the various events held throughout the competition, and did so with "good intentions."
"I don't think anybody was trying to do anything underhanded or be unfair. I would think that the people in San Diego (Lambert's hometown) have the same resources available to them that the people in Conway do. I don't know that they were as organized as we were in terms of their support."
"Idol" has been inconsistent in its disclosure of vote totals: Last year, Seacrest said during the finale that 12 million votes was the difference between winner David Cook and runner-up David Archuleta. Other results have been vaguely referenced as "close," while some - like this year's finale - have not been officially addressed at all.
For his part, Lambert said he didn't know the results of the final vote, and "didn't ask." But he was more suspicious of the "Gokey factor" - the migration of church leader Danny Gokey's votes to the more conservative Allen - than any kind of grassroots effort.
"I knew that (losing) was a possibility," Lambert said Tuesday. "I knew that Kris had just as much of a chance as I did. You know, it was gonna come down to the two of us, and the people that had been voting for more of the conservative choices up until then were probably going to default more to Kris than to me."