Ammonium nitrate-based explosives (searchwere used in four deadly homicide truck bombings in Istanbul, police said Thursday, as the bodies of two British diplomats were flown home after a somber ceremony.

Used as fertilizer, ammonium nitrate can become a powerful explosive when combined with fuel oil, a mixture used to make the bomb that killed 168 people in Oklahoma City (search) in 1995.

"The suicide bombers have carried out the attacks with ammonium nitrate-based explosives placed in the beds of pickup trucks," Istanbul's deputy police chief Halil Yilmaz told a news conference.

Western and Turkish officials say the series of homicide attacks in Istanbul (search) — targeting two synagogues Nov. 15, and two British targets five days later — bore the hallmarks of Usama bin Laden's Al Qaeda (search).

A Turkish court on Thursday placed two suspects under arrest and charged them with membership in an illegal organization, which carries a maximum of five years in prison, the Anatolia news agency said. So far, a total of 20 people have been charged in the investigation.

Seven people were released from custody after questioning Thursday, while two others were taken in for interrogation, Anatolia reported.

The series of attacks killed 61 people, including the four homicide bombers, and wounded 712, Yilmaz said in a statement to the Anatolia news agency.

Yilmaz, earlier in the day had announced the death toll as 55, including the four bombers. But a new statement from him raised the figure, the semiofficial agency said. Police did not provide an explanation regarding the discrepancy. But authorities have been struggling to identify body parts for days.

British Consul-General Roger Short and his assistant, Lisa Hallworth, were among those killed when an Islamic militant rammed an explosive-laden pickup truck into the main gate of the British Consulate. That attack happened minutes after a homicide bomber exploded a truck outside a London-based bank — the kind of near simultaneous, multiple attacks carried out by Al Qaeda in the past.

Turkish soldiers loaded their coffins, each draped with a Union Jack, onto a Turkish military cargo plane for the trip to Britain. Istanbul Gov. Muammer Guler joined other British diplomats for a religious ceremony at Ataturk Airport.

Yilmaz said two Japanese-made Isuzu pickup trucks were used in the synagogue attacks, while two white Japanese-made Hino pickup trucks used to attack the British targets.

Police raiding suspects' houses confiscated two handguns and two "pen guns" — which can fire a single bullet — four shotguns and sniper's binoculars. They also seized bomb-making material, tear gas, ski masks, 20 wireless radios, cameras, and documents in Arabic, Yilmaz said.

Turkish officials have said all four homicide bombers were Turkish nationals, militants with international contacts. Newspapers have said some of them could have been trained in Al Qaeda camps in Afghanistan or Iran.

"Their foreign connections have been found out. They have been to Al Qaeda. There are pro-Chechens among them," Justice Minister Cemil Cicek was quoted as saying by the newspaper Milliyet on Thursday. Cicek did not elaborate.

Police raids of suspected hideouts of Islamic militants were under way. Despite at least three claims of responsibility allegedly from Al Qaeda, the prime minister said investigators had not reached any conclusions.

Police also are investigating possible links between the bombers and Hezbollah (search), an illegal Islamic group that is different from the Lebanon-based group with the same name. Three of the homicide bombers were from the southeastern town of Bingol — a hotbed of Hezbollah activity.