The militants turned off all the lights inside the Holey Artisan Bakery around an hour after they stormed the upscale café in the capital's Gulshan.

As darkness descended on the café, she heard someone groaning, and it sent a shudder down her spine.

It sounded as if an animal was being slaughtered, one of the survivors of the café attack told an investigator, giving an account of the ordeal she had gone through on July 1 night.

At one stage, one of the militants told her to identify herself. And when she did, he asked her to go to a room where several other survivors had been confined, the investigator said, quoting the survivor. 

The militants brutally killed 20 hostages, including nine Italians, seven Japanese and an Indian citizen. 

The female survivor, along with 12 others, were freed on July 2 morning following a commando operation that ended the over 11-hour siege.

Five terrorists and a chef of the café were killed in the operation. Another injured staff later died in hospital. Two police officers were also killed in their bid to end the standoff.

Two months into the attack that left a deep scar on the nation's psyche, the investigators claim they have made significant progress in the probe.

“The terror attack was discussed globally. With all our capacity, we are trying to carry out a probe of international standard. So, it will take a bit more time than usual investigations,” Monirul Islam, chief of Counter Terrorism and Transnational Crime Unit, told The Daily Star on Tuesday.

“We have already made significant progress in the investigation.”

Monirul said their probe focuses on finding the motive behind the attack, the sources of the terror fund, arms and explosives used in the attack, the people who coordinated it, the recruiter of the five attackers, their trainers and those who helped the attackers rent places for using those as hideouts in the capital.

“The motive behind the attack is already clear,” Monirul, the lead investigator, said without elaborating.

“We have already identified around 10 people who, at different stages, played roles in bringing the firearms and explosives to Dhaka after those were smuggled into Bangladesh through Chapainawabganj border,” said Monirul, also additional commissioner of Dhaka Metropolitan Police.

Following the Gulshan attack, law enforcers recovered three AK-22 semiautomatic rifles, seven AK-22 magazines and 79 bullets from the café. Five 9mm pistols, 50 bullets and around 200 bullet shells were also found there.

The AK-22s were first smuggled to Nepal from where those were taken to India, and then the arms were brought to Dhaka through Chapainawabganj border, said the investigators.

The pistols were smuggled in from Bihar's Munger district in India through the same border, which was also used for bringing in the explosive materials, including power gel and detonators, they said.

Monirul said they have already identified some of the people who recruited and trained the attackers, and also eight to 10 others who played roles, at various stages, in carrying out the attack.

The investigation has revealed that slain terror kingpin Tamim Chowdhury masterminded the attack. He even bade farewell to the five terrorists in the city's Bashundhara before they attacked the café.

Nurul Islam Marzan, a “top-level militant organiser”, was the “operation commander” of the café attack.

During the siege, the attackers sent photos and reports to Marzan who then sent those to Tamim. The photos were published by the propaganda wings of global terror outfit Islamic State, said the investigators.

A huge amount of money was spent on training the attackers, renting apartments for them in the city, and purchasing arms and explosives for the attack, they added.

About the source of the terror fund, an investigator said they already found that Tk 13.40 lakh was sent to “Neo JMB” coordinator Tamim by a sympathiser from abroad. The transaction was made via an illegal channel through a South Asian country, known as a transit point of illegal money.

Police have shown Hasnat Karim, former teacher at a private university and a survivor of the attack, arrested in the case filed over the bloody café siege.