The Department of Homeland Security (DHS) has discovered a plot to poison food in U.S. hotels and restaurants, CBS News reports.

The plot, uncovered earlier this year, involved the simultaneous poisoning of food served in hotels and restaurants across the U.S. over a single weekend. Two different poisons, ricin and cyanide, would be slipped into salad bars and buffets.

According to CBS, a "key intelligence source" confirmed that the threat was "credible." So credible, in fact, that the DHS, Department of Agriculture and the FDA briefed key security officers in the hotel and restaurant industries about the attack.

Officials believe that those behind the plot may be tied to the Yemeni-based al-Qaida in the Arabian Peninsula (AQAP). In October, AQAP attempted a cargo bombing scheme, in which explosives were placed in two packages shipped to Chicago addresses.

On Monday, Department of Homeland Security spokesman Sean Smith refused to respond specifically to the CBS report. "However," Smith said, "counterterrorism and security communities have engaged in extensive efforts for many years to guard against all types of terrorist attacks, including unconventional attacks." He noted that al-Qaida has stated it hopes to carry out unconventional attacks for more than a decade and that AQAP has voiced similar intentions over the past year.

Tuesday, Attorney General Eric Holder told ABC News that he is trying to "make people aware of the fact that the threat is real, the threat is different, the threat is constant."

In the poisoning case, it appears officials stopped it from coming to fruition. Yet, Holder's warning Tuesday rings clear - "The terrorists only have to be successful once."