It began as a grudge, though one man supposedly insists it was all "a joke."

Two men suspected of smuggling Chinese immigrants across the Mexican border placed a cell phone call warning of a possible terror threat against Boston, Mexican and American law enforcement officials said.

The call set authorities in motion — alerting the public, increasing security at the airport, on the subway. Gov. Mitt Romney (search) even skipped President Bush's inauguration to return to Boston.

On Tuesday, the FBI said the chilling tip was a false alarm.

"There were, in fact, no terrorist plans or activity under way," the statement said. "Because the criminal investigation is ongoing, no further details can be provided at this time."

Jose Ernesto Beltran Quinones (search), one of 16 people sought for questioning about the alleged terror plot, was detained over the weekend in Mexicali, a border town near San Diego. His son, also named Jose, was detained Monday.

Mexico's federal Attorney General's office released a statement late Tuesday saying Beltran, 34, had admitted calling 911 on Jan. 17 from a cell phone to report the fake threat.

Beltran was under the influence of alcohol and drugs, the statement said; he insisted the call "was only a joke."

Beltran also denied being a migrant smuggler. He was released because of a lack of evidence against him, although the investigation into the matter will continue, the statement said. The news release did not indicate when Beltran was detained.

According to a law enforcement official in the Baja California Attorney General's Office, the two men were involved in smuggling Chinese migrants across the border and told investigators smugglers had squabbled over a deal, and that one had anonymously called in the false tip to U.S. authorities as revenge.

The source, who asked not to be named, did not say which smuggler had made the call.

The two were later released because they had obtained a court injunction preventing their arrest. Relatives at their houses told reporters Tuesday they were not at home.

The FBI statement did not say whether Quinones and his son had provided the information that allowed the threat to be ruled out, but the bureau did thank Mexican law enforcement agencies for their help.

Almost immediately, officials stressed that they doubted the credibility of the terror tip. A leading theory had been that the plot was called in to exact revenge on Chinese immigrants that failed to pay for being taken across the border.

The tipster claimed members of the group had talked about material supposedly called "nuclear oxide" (search) that would follow them from Mexico to Boston. The implication was the group was plotting to detonate a radioactive "dirty bomb" that spews hazardous material and can sicken or kill people.

Federal authorities released the names and photos of four Chinese nationals being sought for questioning — and a few days later determined that one of them had been in federal custody for more than two months and had no terrorist connections.

"While we questioned the credibility of the source information from the very beginning, we were determined to run this out, as we always do, to ensure there was no threat," Massachusetts U.S. Attorney Michael Sullivan (search) and Boston FBI agent in charge Kenneth Kaiser said in a statement.

Romney's office referred calls to the state's Public Safety Office. Katie Ford, an agency spokeswoman, said authorities were concerned that unfounded terror alerts would make it look like investigators were "crying wolf."

"We are keenly aware that in situations like this, government and the media are in a no-win situation," she said. "We don't want to desensitize the public to potential terror attacks."