Efforts to permanently plug the world’s largest offshore oil spill reached a milestone when BP Plc’s crucial relief well reached its target — the blown-out Macondo well that began spewing oil almost five months ago, a U.S. official said.

Now that the relief well has intersected with BP’s well, workers have an opening through which they can pump in mud and cement to kill the Macondo well for good.

“The aggregate data available supports the conclusion that the two wells are joined,” retired Coast Guard Admiral Thad Allen, the top U.S. official overseeing the spill response in the Gulf of Mexico, announced late Thursday in a statement.