Bahraini authorities have arrested five suspects in connection with a bombing that killed two policemen last month in the Gulf kingdom, and accused them of having links with Iran, the interior ministry said in a statement on Thursday.

Police chief Major General Tariq al-Hasan said the suspects had links with Iran’s Revolutionary Guards and Iran-backed Lebanese militant movement Hezbollah – no evidence, however, was produced to substantiate the claim.

The suspects were linked to other “terrorist incidents in Bahrain in recent years,” he said in the statement.

The 28 July bombing killed two Bahraini policemen and wounded six others on Sitra island outside the capital Manama, an area often the scene of pro-democracy protests.

The blast came just days after Bahraini authorities claimed they had foiled an attempt to smuggle weapons from Iran.

Hasan said that forensic tests showed that explosives used in the Sitra bombing were similar to substances seized in the smuggling attempt.

The suspects include a 24-year-old identified as Mohammed al-Tooq, who allegedly planted and detonated the bomb, the ministry said.

He “is known to have spent time in Iran following his role in a deadly terrorist attack in 2013 which killed a policeman,” it said. It was not clear how the ministry obtained the information about Tooq’s time in Iran.

He was also accused of having trained at a Hezbollah camp.

“This is yet another disturbing incident in which Iranian actions are attempting to undermine Bahrain’s security and stability,” Hasan said.

The tiny but strategic US ally has seen frequent unrest since a popular uprising erupted four years ago demanding a constitutional monarchy.

Bahrain has frequently accused Tehran of backing the country’s pro-democracy movement but has rarely backed up their claims by publishing evidence.

The pro-democracy uprising has been increasingly categorised as the island’s Shia majority community rebelling against the Sunni ruling Khalifa family.

However, many commentators have warned against propagating a sectarian narrative.

Some have pointed to activists like Leftist Ibrahim Sharif, a Sunni, who has been imprisoned repeatedly as evidence of a far-reaching crackdown by authorities that targets those calling for political reform, rather than a reaction to a schism between Bahrain’s Shia and Sunni Muslims.