No one was in charge of planning and security decisions for the July 13 campaign rally where former President Donald J. Trump was shot, according to a preliminary report released Wednesday by a Senate committee that described a withering list of Secret Service failings.

Diffuse and blurred leadership roles for the event in Butler, Pa., led to communications breakdowns and security lapses, the Senate Homeland Security and Governmental Affairs committee found. There was confusion over who was supposed to secure the building from which the gunman opened fire. There were multiple plans in place, none of them definitive. There were urgent warnings that were picked up but then dropped.

The report painted a portrait of hapless on-site leadership unaware of potential threats to Mr. Trump’s safety and a culture within the agency of individuals unwilling to take responsibility for those failures. Even after many hours of testimony, the committee said that no one involved in the rally’s security plans could say who made the call to exclude from the security perimeter a set of nearby warehouses, one of which the gunman eventually climbed onto and used as a perch to shoot at Mr. Trump.

“Everybody points fingers at someone else,” Senator Rand Paul of Kentucky, the top Republican on the panel, said in remarks to reporters on Tuesday.