Colombo, Sri Lanka (CNN) Police in Sri Lanka have warned that more suspects in the Easter Sunday bombings were on the run, as it emerged that a prominent spice tycoon was being held on suspicion of helping two of his sons who participated in the attacks.

Authorities in Colombo issued photographs of a number of suspects, but were forced to withdraw one of the images when it transpired that it was of someone who had nothing to do with the bombings.

Adding to the sense of confusion, the government significantly lowered the death toll , from 359 to 253.

A senior government official told CNN that one of the bombers, Ilham Ahmed Ibrahim, had previously been arrested and released. "It was the suicide bomber of the Cinnamon Grand bomb attack who was released earlier," the official said, referring to one of the hotels blown up by the attackers.

The official did not provide further details, and CNN was unable to contact court officials late Thursday to confirm the arrest.

One of Ibrahim's brothers, Imsath, also blew himself up as part of the coordinated terror wave, according to two sources with knowledge of the investigation. Their father, Mohamed Yusuf Ibrahim, who was detained in the wake of the attacks, was suspected of aiding and abetting his sons, police spokesman Ruwan Gunasekera told CNN.

Spice trader Mohamed Ibrahim, center, looks on as his son Imsath, right, shakes the hand of a government minister, in a 2016 photo.

Mohamed Ibrahim is the founder of Colombo-based Ishana Exports, which describes itself on its website as the "largest exporter of spices from Sri Lanka since 2006." Other members of the Ibrahim family known to police have also been detained. CNN has not been able to reach Mohamed Yusuf Ibrahim or other family members for comment.

Adding to the ongoing turmoil in the aftermath of the attacks, Sri Lanka's Defense Secretary Hemasiri Fernando submitted his letter of resignation on Thursday, presidential adviser Shiral Lakthilaka said. The government has previously acknowledged and apologized for failing to act on warnings received from Indian intelligence before the bombings.

Meanwhile lawyers for four men arrested in January north of Colombo, in connection with an explosives plot, said that none were involved in Sunday's attack. Ishmail Mohammed Naleem, a lawyer for the group, told CNN that all four were still alive, and that any suggestion they were involved in the Easter Sunday bombings was untrue.

A security officer stands guard outside St. Anthony's Shrine where bombing was carried out on Easter Sunday, in Colombo.

Bomb designs found

Sri Lankan authorities found designs for bombs in a house they raided on Sunday in the Panadura district, south of Colombo, according to a high-ranking local police officer. Police also found batteries and packaging for ball bearings.

No-one in the house when it was raided. The house had been rented out to new tenants around two months ago, the officer said. Police and neighbors say the new occupants offered about twice the normal asking price to rent the property.

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The house was sealed as investigators worked inside on Thursday. CNN saw men identified by local police as FBI agents leaving the house.

A taxi carrying one of the bombers to the Shangri-La hotel in Colombo stopped at a house in Panadura on Sunday, a senior intelligence official told CNN. The taxi had taken the bomber from a mosque in in Dharga Town, further south, which had links to National Tawheed Jamath, the group suspected of being involved in the attack. It was not clear whether the house where the taxi stopped in Panadura was the same as the one where the bomb designs were found.

When the taxi reached Panadura, the passenger asked the driver to remove the back seat to make room for boxes, the intelligence source said.

During the rest of the drive to the Shangri-La hotel, the passenger instructed the driver to drive slowly and carefully, saying he didn't want his "clothes" in the box to get crushed, according to the intelligence source. Once the car arrived at the Shangri-La, the passenger did not allow the driver to help remove the boxes from the car, instead carrying them into the hotel unaided.

International operation

Across Sri Lanka, a huge international criminal investigation was ramping up, with six foreign police agencies and Interpol assisting local police, including Scotland Yard from the UK and the FBI from the US.

Gunasekera, the police spokesman, said officers from Sri Lanka's Criminal Investigation Department (CID) and Terrorism Investigation Department (TID) had raided five safe houses across the country in connection with Sunday's attacks. Those locations have since been sealed for forensic investigation.

More than 70 suspects have been taken into custody on a range of charges, including suspicion of terrorism, aiding and abetting terrorism and conspiracy to commit terrorism, Gunasekera said. Four high-level suspects are being held by TID, and 33 are being held by CID, he added.

Of those arrested, four suspects are female, and all are Muslims. Gunasekera said most of them are family members and friends of the suspected suicide bombers. None of those arrested are foreigners.

A soldier stands guard outside St. Anthony's Shrine in Colombo, one of the Easter Sunday targets.

Significant raids were carried out on Wednesday night, he added, in which 16 people were arrested at various locations, most near the capital Colombo. Three shotguns and two walkie-talkies were also seized.

As police continue to investigate how a previously little-known terror group managed to pull off a huge and coordinated series of attacks, Gunasekera also revealed that police had confirmed that an explosion in the predominantly Muslim area of Kathankudi, in eastern Sri Lanka, in early April was a test run by the terrorists. In that explosion they blew up a motorcycle.

More search operations were underway across Colombo, and roadblocks had been set up. Police have asked the public not to panic, a spokesman told CNN. The country remains on high alert and numerous controlled demolitions have been carried out of suspicious packages and vehicles in recent days.

Prime Minister Wickremesinghe warned of the potential for more attacks in the country. In his CNN intevriew, he said authorities were targeting "sleepers" -- terrorists who could activate to initiate another round of attacks.

"Police and security forces are rounding up those involved, but they're also rounding up the sleepers, those used on second and third rounds (of attacks)," he said.

This story has been updated to reflect later developments.