A cap collected some of the oil spewing out of the blown-out Gulf well, but black crude was still leaking into the sea, and officials said they won’t know until later on Friday how the device is working.

With BP declaring failure in its latest attempt to plug the uncontrolled gusher feeding the worst oil spill in US history, the company is turning to yet another mix of risky undersea robot maneuvers and longshot odds to keep crude from flowing into the Gulf.