Calcutta, March 26: A cellphone the gang had used as a torch inside the dark rooms of the Ranaghat school has shown light on the investigation that finally led police to the two suspects who were arrested today.

When the police saw CCTV footage of one of the gang members using a cellphone torch, they got in touch with cellular service providers for details of the nearest cellphone tower's call dump - a digital compilation of calls that originate or terminate through a particular tower.

Investigators said that studying the cellphone records for several days after the March 14 rape and robbery, they tracked the whereabouts of at least five suspected gang members.

"They were carrying cellphones. One of the phones was seen on CCTV footage being used as a torch. We started working on the digital trail," said an investigator who refused to divulge further details.

Sources in the home department said the investigators received a few thousand pages of call records from the telecom service providers. The investigators sorted out five numbers whose tower locations were changing from time to time, the sources said.

"All five numbers were moving together. Their tower locations were plotted in places such as Kharagpur in West Midnapore, Balurghat in South Dinajpur and Englishbazar in Malda," a source said.

However, instead of trailing the five "moving" phones, the sleuths started tracking the people with whom these numbers were in touch. This is how the CID stumbled upon at least three numbers - at Habra in North 24-Parganas, Mumbai and Uttar Pradesh.

The Habra number took the investigators to Gopal Sarkar, a Bangladeshi national who has been illegally living in Bengal since 2002.

"A mason by profession, Sarkar claimed to have invited the men, whose numbers were tracked, to his house to attend a marriage ceremony a few days before the incident. Further investigations revealed that he had harboured Bangladeshi nationals earlier too. With their base at Sarkar's home, the gang would operate in various parts of Bengal," an investigator said.

The preliminary investigation has revealed that on the days the gang was missing from Sarkar's home, robberies with modus operandi similar to the one in Ranaghat, like overpowering the guard, were reported in parts of West Midnapore and Malda. This prompted the police to suspect if the same gang was behind the robberies.

From Habra police, CID sleuths learnt that on March 12, two days before the attack at the Ranaghat school, the gang had been charged with disorderly conduct. The police had acted on complaints of nuisance reported by Sarkar's neighbours.

The investigators tracked down the Mumbai number and reached Mohammad Salim Sheikh. He was arrested from Mahila Milan Jhopar Patti in the Nagpada police station area. It is suspected that Salim was present during the heist as his face reportedly resembles the person identified as suspect number three in the CCTV footage.

Salim, according to CID sources, was the head of the gang and was being referred to as "boss" by the rest of the members. Salim had carried out crimes in Bengal earlier too, with Sarkar's house acting as the base, the sources said.

The sources said Salim, who is "not in the habit of taking instructions", is suspected to have brutalised the nun. Sarkar has told the investigators that after the Ranaghat rape-and-robbery, he had seen off Salim at Howrah station, from where the gang leader boarded a train to Mumbai.

A CID team that went to Mumbai took the early morning flight today to Calcutta. Salim was produced in the additional chief judicial magistrate's court in Ranaghat and remanded in 14 days' CID custody.

The investigators have learnt that some of the gang members, after fleeing the Ranaghat school, had boarded a train to Gede and sneaked into Bangladesh through the porous border. Others had fanned out across the country.

Apart from the digital clues, the CID also acted on intelligence inputs received from border areas.

Sources said the investigators questioned multiple touts who are known to illegally ferry people across the border. These touts apparently identified three of the suspects whose faces were captured by the CCTV cameras.

According to the touts, all three had crossed the border after the incident.

Assistant public prosecutor Pradip Pramanik said: "It is a big success for the CID team. Considering the international focus on the incident and based on the case diary, we prayed for 14 days' custody for the accused and the court granted it. Salim would again be produced in court on April 9."