A top police official on Saturday acknowledged possible security lapses that allowed an assassin to fire his gun into former Japanese Prime Minister Shinzo Abe while he was addressing a campaign rally, raising questions how could the attacker get so close behind him.

Abe was shot in the western city of Nara on Friday and airlifted to a hospital but died of blood loss. Police arrested the attacker, a former member of Japan’s navy, at the scene. Police confiscated his homemade gun and several others were later found at his apartment.

The attacker, Tetsuya Yamagami, told investigators he acted because he believed rumors that Abe was connected to an organization that he resents, police said. Japanese media reported that the man had developed hatred toward a religious group that his mother was obsessed about and that caused his family financial problems. The reports did not specify the group.

On Saturday, a black hearse carrying Abe’s body and accompanied by his wife, Akie, arrived at his home in Tokyo’s upscale residential area of Shibuya. Many mourners, including top party officials, waited for his remains and lowered their heads as the vehicle passed.

Nara prefectural police chief Tomoaki Onizuka said Abe’s assassination was his “greatest regret” in a 27-year career.

“I cannot deny there were problems with our security,” Onizuka said. “Whether it was a setup, emergency response, or ability of individuals, we still have to find out. Overall, there was a problem and we will review it from every perspective.”

Abe’s assassination ahead of Sunday’s parliamentary election shocked the nation and raised questions over whether security for the former prime minister was adequate.

Some observers who watched videos of the attack noted a lack of attention in the open space behind Abe as he spoke.

A former Kyoto prefectural police investigator, Fumikazu Higuchi, said the footage suggested security was sparse at the event and insufficient for a former prime minister.

“It is necessary to investigate why security allowed Yamagami to freely move and go behind Mr. Abe,” Higuchi told a Nippon TV talk show.

Experts also said Abe was more vulnerable standing on the ground level, instead of atop a campaign vehicle, which is usually the case but was reportedly unavailable due to his hastily arranged visit to Nara.

Read: Japan's ex-leader Shinzo Abe assassinated during a speech

“Looks like police were mainly focusing on frontward, while paying little attention to what’s behind Mr. Abe, and nobody stopped the suspect approaching him,” said Mitsuru Fukuda, a crisis management professor at Nihon University. “Clearly there were problems.”

Fukuda said that election campaigns provide a chance for voters and politicians to interact because “political terrorism” was extremely rare in postwar Japan. But Abe’s assassination could prompt stricter security at crowded events like campaigns, sports games and others.

During a parliamentary debate in 2015, Abe resisted suggestions by an opposition lawmaker to beef up his security, insisting that “Japan is a safe country.”

In videos circulating on social media, the 41-year-old Yamagami can be seen standing only a few meters (yards) behind Abe across a busy street, and continuously glancing around.

A few minutes after Abe stood at the podium and started his speech — as a local party candidate and their supporters stood and waved to the crowd — Yamagami can be seen taking his gun out of a bag, walking toward Abe and firing the first shot, which released a cloud of smoke, but the projectile apparently missed Abe.

As Abe turned to see where the noise came from, a second shot went off. That bullet apparently hit Abe’s left arm, missing a bulletproof briefcase raised by a security guard who stood behind him.

Abe fell to the ground, with his left arm tucked in as if to cover his chest. Campaign organizers shouted through loudspeakers asking for medical experts to provide first-aid to Abe. His heart and breathing had stopped by the time he was airlifted to a hospital, where he later pronounced dead.

Police on Saturday said autopsy results showed that a bullet that entered Abe’s upper left arm damaged arteries beneath both collar bones, causing fatal massive bleeding.

According to the Asahi newspaper, Yamagami was a contract worker at a warehouse in Kyoto, operating a forklift. He was described as a quiet person who did not mingle with colleagues. A next-door neighbor at his apartment told Asahi he never met Yamagami, though he recalled hearing noises like a saw being used several times late at night over the past month.

Read: Assassination of Japan’s Shinzo Abe stuns world leaders

Japan is particularly known for its strict gun laws. With a population of 125 million, it had only 10 gun-related criminal cases last year, eight of then gang-related.

Even though he was out of office, Abe was still highly influential in the governing Liberal Democratic Party and headed its largest faction. But his ultra-nationalist views made him a divisive figure to many.

Abe stepped down two years ago blaming a recurrence of the ulcerative colitis he’d had since he was a teenager. He said he regretted leave many of his goals unfinished, especially his failure to resolve the issue of Japanese abducted years ago by North Korea, a territorial dispute with Russia, and a revision of Japan’s war-renouncing constitution.

That ultra-nationalism riled the Koreas and China, and his push to create what he saw as a more normal defense posture angered many Japanese liberals. Abe failed to achieve his cherished goal of formally rewriting the U.S.-drafted pacifist constitution because of poor public support.

Loyalists said his legacy was a stronger U.S.-Japan relationship that was meant to bolster Japan’s defense capability. Abe divided the public by forcing his defense goals and other contentious issues through parliament.

Chinese leader Xi Jinping, who early on had a frosty relationship with Abe, sent a condolence message to Japanese Prime Minister Fumio Kishida on Saturday, a day after most other world leaders issued their statements.

Xi credited Abe with making efforts to improve China-Japan relations and said he and Abe had reached an important understanding on building better ties, according to a statement posted on China’s Foreign Ministry website. He also told Kishida he is willing to work with him to continue to develop neighborly and cooperative relations.

Abe was groomed to follow in the footsteps of his grandfather, former Prime Minister Nobusuke Kishi. His political rhetoric often focused on making Japan a “normal” and “beautiful” nation with a stronger military through security alliance with the United States and bigger role in international affairs.

He became Japan’s youngest prime minister in 2006, at age 52, but his overly nationalistic first stint abruptly ended a year later, also because of his health, prompting six years of annual leadership change.

He returned to office in 2012, vowing to revitalize the nation and getting its economy out of its deflationary doldrums with his “Abenomics” formula, which combines fiscal stimulus, monetary easing and structural reforms. He won six national elections and built a rock-solid grip on power.