Shakil died at Samdado Japanese Cuisine, a restaurant on Road 35 in Gulshan 2 on Tuesday afternoon.
Gulshan police Officer-in-Charge (Investigation) Salauddin confirmed the matter to the Dhaka Tribune.
The prime minister's personal secretary Ihsanul Karim, political adviser HT Imam, and Awami League Joint General Secretary Mahabubul Alam Hanif were also on the scene.
While talking to the media, Awami League Office Secretary Abdus Sobhan Golap said Shakil had gone to the restaurant around 8pm Monday.
He said Shakil's body has been taken to Birdem Hospital.
Golap says the initial inquest identified the cause of death as cardiac arrest.
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Police and senior government officials crowd the premises of Samdado Japanese Cuisine in Gulshan 2 Mahmud Hossain Opu/Dhaka Tribune
6 restaurant staff detained
However, the restaurant staff are claiming that Shakil may have died overnight.
According to their claims, Shakil had gone to Samdado at 4pm on Monday. He was joined by a close friend, Nur Mohammad Biplob, around 5pm.
The claims were corroborated by Biplob’s driver Abdul Barek.
The restaurant staff and Barek said Shakil and Biplob would frequently dine at the restaurant, and their Monday visit seemed no different. Around 9pm, Shakil said he was feeling sick and Biplob offered him a ride home but his offer was turned down. Biplob left soon after and the restaurant staff allowed Shakil to recuperate on the restaurant premises.
Meanwhile, police have detained six restaurant staff including the manager from the restaurant for further questioning.
OC Salauddin informed the Dhaka Tribune that Samdado has been closed down temporarily until further notice.
Grief pours in
Prime Minister Sheikh Hasina expressed her profound shock and condolences at the death of her special assistant.
In a message, she wrote: "With Shakil's demise, the nation has lost an efficient organiser and an admirer of Bangabandhu. As a student leader in particular he had immense contribution to country's democratic movements."
She recalled his contribution to the cultural arena as a student leader, poet and writer.
Sheikh Hasina prayed for "Eternal peace of the departed soul and expressed profound sympathy to the bereaved family members."
Speaker Shirin Sharmin Chowdhury, Deputy Speaker M Fazle Rabbi Mia and Chief Whip ASM Firoz, Public Administration Minister Syed Ashraful Islam, Cultural Affairs Minister Asaduzzaman Noor, Expatriate Welfare Minister Nurul Islam, Information Minister Hasanul Haq Inu, Land Minister Shamsur Rahman Sharif, and State minister for Land Saifuzzaman Chowdhury also expressed their condolences.
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Awami League Joint General Secretary Mahbubul Alam Hanif seen arriving at the spot Mahbub Hossain Opu/Dhaka Tribune
Funeral on Wednesday
State Minister for Power, Energy, and Mineral Resources Nasrul Hamid said the body will be moved to Dhaka Medical College Hospital for autopsy on Wednesday morning.
Following the autopsy, the namaz-e-janaza will be held at Dhaka University Central Mosque on 11am Wednesday.
Later, he will be buried at his birthplace Baghmara in Mymensingh.
A former Bangladesh Chhatra League leader, Shakil is survived by a wife and a daughter.
Shakil was born in 1968 to Jahurul Hoque, who is the administrator of the local government council in Mymensingh. He graduated from Dhaka University with a degree in social science.