Iranian authorities have arrested four more suspects after detaining a lone gunman in the killing of at least one person at a Shia shrine, state media reported on Monday.

The attack came less than a year after a similar one on the same holy site, the Shah Cheragh mausoleum in Shiraz, the capital of Fars province in Iran’s south.

The official IRNA news agency quoted Fars provincial chief justice Kazem Mousavi as saying that “four suspects have so far been arrested for links to the attack.”

The four are in addition to the gunman whose arrest was announced on Sunday night by Fars commander of the Islamic Revolutionary Guards Corps, Yadollah Bouali, who spoke on state TV.

In addition to the one fatality, eight people were wounded, IRNA reported.

There was no immediate claim of responsibility but Fars provincial governor Mohammad Hadi Imanieh blamed the militant Islamic State group.

He told state TV that the assailant sought “to take revenge for the execution of two terrorists” convicted of carrying out a similar attack last year.

On October 26, a mass shooting at the shrine left 13 people dead and 30 wounded. IS later claimed the attack.

Iran hanged two men in public on July 8 over the killings after their conviction for “corruption on earth, armed rebellion and acting against national security,” the judiciary’s Mizan Online news website said.

Three other defendants in the case were sentenced to prison for five, 15, and 25 years for being members of IS, Moussavi said.

In November, Tehran said 26 “takfiri terrorists” from Afghanistan, Azerbaijan, and Tajikistan had been arrested in connection with the mass shooting.

In Iran, the term takfiri generally refers to militants.

The Shah Cheragh mausoleum is home to the tomb of Ahmad, brother of Imam Reza — the eighth Shia imam — and is considered the holiest site in southern Iran.

Last year’s shooting occurred as nationwide protests gripped Iran following the death in custody of Mahsa Amini, 22, arrested for an alleged breach of strict dress rules for women.