But tight-lipped interrogators declined to divulge any details of the quizzing with Inspector General of Police (IGP) Abdul Quiyyum telling newspersons, "Everything relevant is being asked."

Director General of Rapid Action Battalion (Rab) Abdul Aziz Sarker echoed him without detailing. "We're asking him all sorts of things he has done so far and cross-checking the information he is providing," he told The Daily Star last night over phone.

"Rahman is the supremo of an organisation that threatened the entire country," he said, adding, they need some time to get every thing out of him.

Clues to many killings in the country could be found out after quizzing the chief of outlawed Jama'atul Mujahideen Bangladesh (JMB), said the IGP yesterday in an interview with BBC Bangla Service.

Referring to a number of cases filed with different police stations across the country since the August 17 blasts, he said, Rahman's complicity was learnt during investigations. "We also need hard pieces of evidence to prove those."

Asked if any murder case will be filed against Rahman, the IGP said, "Rahman was not physically present on the spots, but evidence showed that everything was done at his diktat."

"His [Rahman's] name also came out in the killings and attacks in Jhalakathi, Chittagong and Gazipur," he said. Rahman is accused in 40 charge sheets submitted by police so far.

"We think we can bring the highest conviction against him," the IGP said.

Earlier on March 1, the man, who terrorised Bangladesh with ruthless squads of bombers and boasted of suicide instead of surrendering, walked out of his Sylhet hideout into law enforcers' hands, ending 34 hours' suspense.

He was brought to Dhaka by road in high security for interrogation at the Rab-1 office in Uttara after a Sylhet court remanded him for 10 days the same day.

Investigators hope the arrest of Rahman will lead to the revelation of a comprehensive picture of JMB including its network, fronts, operations and links to organisations at home and abroad.

"There's no doubt he holds the key to many important things. But we'll be able to say the exact extent of his usefulness in uncovering the mysteries only after interrogating him thoroughly," Rab intelligence chief Lt Col Gulzar Uddin Ahmed told The Daily Star earlier.

Four members of Majlish-e-Shura, the seven-member highest tier of JMB, arrested so far have all named Rahman as the man, who knows every detail of JMB, Gulzar added.

Investigators believe if Rahman's statements during interrogation give them leads to trap the so-called Bangla Bhai and other few top militant leaders, activities of JMB could well be contained.

Asked if Interpol's assistance will be sought to quiz Rahman, the IGP told the BBC that Rab and police have showed much success in comparison with investigators of some developed countries. "We are open if it's necessary."