Masoon Billah, who headed the death squad of the LeJ, admitted to planning the attack on Justice Maqbool Baqar on June 26 in the busy Burns road area of the city, police said.
"Billah alias Abu Bakr has claimed that the planning of the attack took place at accomplice Bashir Leghari's house," one official said.
The attack on Justice Baqar killed nine people and injured many more, police said.
The Intelligence Bureau (IB) in its reports sent to the government confirmed they had arrested and detained the suspected chief of the death squad who was also produced in court and remanded to judicial custody until June 29.
Interestingly, the banned militant group Tehreek-e-Taliban had also claimed responsibility for the attack.