A police team deployed at the Moghbazar blast site called the Fire Service on Sunday morning after they had noticed gas was leaking from a pipe there.
A Fire Service team rushed there and found that gas was leaking from a Titas Gas supply line, Fire Service Deputy Director Debashis Bardhan told The Business Standard.
"We are suspecting that gas leaked from Titas supply line and contributed to the fatal blast," said Debashis.
"The line might be illegal. Titas is examining it and they could say whether it is legal or illegal," he added.
He also said there were some other lines in the building and leakage was found there too.
However, Titas Gas General Manager Shafiqul Islam said their team had visited the spot after getting information on gas leakage and found no Titas line there.
Titas Managing Director Ali Iqbal Md Nurullah earlier said there was a Titas line which was disconnected in 2014.
Meanwhile, the Fire Service investigation team on Sunday held a meeting with police, an intelligence agency, and an engineering consultation firm over the blast.
The probe committee is planning to submit its report within two days, officials said.
The preliminary investigation suggested that "leaked gas" was the main reason for the explosion that occurred at the building opposite to the Moghbazar branch of Aarong on Shaheed Sangbadik Selina Parvin Road on 26 June.
Police are now analysing which institution and personnel were responsible for the blast, AKM Rahmatullah Chowdhury, member secretary of the Police Headquarters' probe body and also the head of the Bomb Disposal Unit of Counter Terrorism and Transnational Crime, told The Business Standard.
"We are making a three-dimensional visualisation of the incident, which will portray every second of the blast – from explosion to the casualties and afterwards," he said.
"We are analysing why gas accumulated in such space, and why the risky old building was being used for years even though it was damaged inside and outside," Rahmatullah added.
A member of the probe body told TBS that the building owner and the renters – Sharma house, Bengal Meat and other businesses – were operating in the dangerous building.
"The building was renovated two years ago, but still it remained risky. So not only the building owner who set up the illegal gas line will be brought to book, we will also quiz some Titas officials too," he said adding that, "It was a complete mismanagement and massacre."
At least 11 people were killed and several hundred were injured in the explosion that had rocked the area wrecking and damaging many vehicles, buildings and business infrastructure.
The police filed a case on 29 June at the Ramna police station over the explosion, mentioning seven entities and an individual as suspects for their "negligence" that led to the explosion.
The case was transferred to the Counter Terrorism and Transnational Crime unit of the Dhaka Metropolitan Police on 3 July.
The police are yet to arrest or detain anyone over the case.