There were "undeniable" flaws in security for former prime minister Shinzo Abe, the head of police in the area where the leader was assassinated admitted on Saturday, pledging an investigation.
Japan's best-known politician was on the campaign trail in the western region of Nara when a gunman opened fire at close range.
Security at local campaign events in Japan can be relatively relaxed, in a country with little violent crime and strict gun laws.
But given Abe's profile, questions have been raised about whether measures to protect him were too lax.
"I believe it is undeniable that there were problems with the guarding and safety measures for former prime minister Abe," Tomoaki Onizuka, head of the Nara prefectural police, told reporters on Saturday evening.
"The urgent matter is for us to conduct a thorough investigation to clarify what happened," he said.
He did not offer any immediate details on specific shortcomings to the security plan, but said he felt a heavy sense of responsibility.
"As the regional police chief responsible for safety and security of the region, I took necessary steps and built structures for security and guarding," he said.
"In all the years since I became a police officer in 1995, in my career that stretches more than 27 years, there is no greater remorse, no bigger regret than this," he said of Abe's death, his voice shaking with emotion.