Several Us Weekly images, both posed and candid, show the couple at various stages of a wedding ceremony at a chapel near Cabo San Lucas, Mexico on Nov. 20. The accompanying story features an interview with the pair, as well as details about the rings, Montag's dress, and the content of their handwritten vows.
The story does not address whether they obtained a marriage license or took part in a separate civil ceremony, which is required by Mexican law to make the union binding.
Their publicist, Rick Rhodes, said he knew nothing about the marriage. "I'm not privy to any of that information," Rhodes wrote in an e-mail. "If there was a wedding I wasn't invited ... Sorry!"
Us Weekly's report, which was picked up by numerous celebrity magazines and blogs, said that none of the couple's friends or relatives was in attendance. Some people close to the couple said they were caught off-guard.
"Everybody's pretty upset," Montag's sister, Holly Montag, told MTV on Monday. Montag's father, Bill Montag, told People magazine that he'd be dismayed if his daughter got married and didn't invite him.
If the story were true, he added, "Then we just have to have another wedding here!"
Us Weekly, which has recently featured the couple in numerous cover stories and photo spreads, reported that they decided to get married while drinking margaritas on the beach. An hour later, they were exchanging vows, the magazine said.
Americans who wish to get married in Mexico must first go through a process that takes about five days, according to Mexico's foreign relations department Web site. They must obtain a health certificate, including blood test results from a local doctor; and provide official translations of legal documents, such as birth certificates.
Only civil, nonreligious marriages conducted in Mexico are valid internationally. Peter Grossman of Us Weekly, who conducted the wedding-day interview with Pratt, 25, and Montag, 22, would only confirm Tuesday that the two exchanged rings and handwritten vows in front of a minister.
"Heidi and Spencer are two people in love who had a beautiful wedding ceremony to celebrate that," Grossman said. "The pictures and words in our issue this week bear that out."
A couple can register their marriage up to 10 days after a ceremony, but California does not recognize marriage ceremonies outside the United States, according to the state's Department of Public Health. A couple can petition a court later to have their marriage legally recognized.
A Tuesday records check in Los Angeles County did not show that the couple had filed for a marriage license, though it can take weeks for such documents to appear public.
Neal Hersh, a Beverly Hills attorney who has handled numerous celebrity divorces said Pratt and Montag's marriage could be legitimate, but only if it was handled properly.
"People can get married anywhere," he said.
It wasn't immediately clear whether MTV captured the wedding ceremony for its show, but an MTV spokesperson said in an e-mailed statement late Tuesday that the network "wishes Heidi and Spencer the best and can't wait to share their special moment with 'The Hills' fans later this season."
Since their relationship came to light on the MTV reality show, Pratt and Montag have been ever camera-ready, willing to pose or invite photographers to capture their "private" moments - including an intimate, champagne-and-roses Valentine's Day celebration aboard a chartered boat earlier this year.
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Mark Stevenson in Mexico City and Anthony McCartney in Los Angeles contributed to this report.