What the couple didn't do, according to the Los Angeles County registrar's office, is file a marriage license after the ceremony.
No big deal, says a member of the singer's camp who declined to be quoted by name.
"They're scheduled to file it next week," she told The Associated Press on Wednesday.
Spears told People magazine essentially the same thing.
"I know we're not completely legal until we file the license, which we'll do next week," the 22-year-old pop star said. "But in a real sense, a spiritual sense, we're married."
Federline, 26, said the delay was caused when the couple moved their wedding date up to Sept. 18 as word leaked out that they were planning to be married on Oct. 16.
Moving the date up a month, he said, didn't give their lawyers time to finish their prenuptial agreement.
"It's done (now)," he said. "The details aren't anyone's business. But it wasn't any big deal."
Kathy Treggs, the Los Angeles County recorder's office manager of public records, said the couple have 10 days from the date of the wedding to file the license. As far as she knows, Treggs said, the couple didn't take out a license from a Los Angeles County office but could have pulled one from any county recorder's office in the state or from any number of notary publics authorized to issue them.
Us Magazine reported that it has obtained a draft of the prenuptial agreement that states if the couple divorce, Spears will pay Federline $300,000 a year for a period of time equal to half the length of their marriage. She will have no financial obligation to his two children by a previous relationship, and any gift valued at $10,000 or more will require a legal document stating who will own it if the couple split.
Federline, described in the document as "presently possessed of property of moderate value," can make no claim to any of Spears' companies but bears no responsibility if any of them lose money.
Spears is worth at least $32 million, according to the document.
The Us Magazine report was skeptical of the motives behind the nuptials.
"Britney Spears is the queen of stunts as we all know," Us Magazine senior editor Albert Lee said on CBS News' Early Show. "She makes Janet Jackson look like an amateur."
Lee theorized that the weekend ceremony may have been a diversionary tactic and that another wedding may be in the offing down the road.
In January, Spears married childhood friend Jason Alexander in a surprise wedding in Las Vegas. That marriage was annulled 55 hours later.