Days after authorities dismissed claims that ISIS brought down a Russian passenger jet, a U. S. intelligence analysis now suggests that the terror group or its affiliates planted a bomb on the plane. British Foreign Minister Philip Hammond said his government believes there is a "significant possibility" that an explosive device caused the crash. And a Middle East source briefed on intelligence matters also said it appears likely someone placed a bomb aboard the aircraft. Metrojet Flight 9268 crashed Saturday in Egypt's Sinai Peninsula after breaking apart in midair, killing all 224 people on board. It was en route to St. Petersburg from the Red Sea resort of Sharm el-Sheikh. The latest U.S. intelligence suggests that the crash was most likely caused by a bomb planted on the plane by ISIS or an affiliate, according to multiple U.S. officials who spoke with CNN. The officials stressed that no formal conclusion has been reached by the U.S. intelligence community and that U.S. officials haven't seen forensic evidence from the crash investigation. Intelligence also suggests someone at the Sharm el-Sheikh airport helped get a bomb onto the plane, one U.S. official said. "This airport has lax security. It is known for that," the official said. "But there is intelligence suggesting an assist from someone at the airport.” Egyptian authorities, who are leading the investigation into the crash, haven't publicly responded to reports on U.S. intelligence. Since the crash, they've downplayed the possibility that terrorism could be involved. The signs pointing to ISIS, another U.S. official said, are partially based on monitoring of internal messages of the terrorist group. Those messages are separate from public ISIS claims of responsibility, that official said. In an audio message from ISIS' Sinai branch that was posted on terror-related social media accounts Wednesday, the organization adamantly insisted that it brought down the flight. "Find your black boxes and analyze them, give us the results of your investigation and the depth of your expertise and prove we didn't do it or how it was downed," the message said. "Die with your rage. We are the ones with God's blessing who brought it down. And God willing, one day we will reveal how, at the time we desire." Typically, ISIS is quick to trumpet how and who carried out any attacks for purposes of praise and propaganda. To some, the fact that ISIS hasn't provided details in this case raises doubt about the group's repeated claims of responsibility. Officials in Egypt and Russia have said there's no evidence to support ISIS' claims. "That was a very baffling way to claim credit for what would be the most significant terrorist attack since 9/11," CNN terrorism analyst Paul Cruickshank said. "But there may have been a method behind this and a reason behind this, and that may have been to protect an insider at Sharm el-Sheikh airport."