An Indian and two Afghan nationals were arrested on Friday, officials said.
The NCB has widened its probe into a recent seizure of about 50 kg of heroin from a house in south Delhi's Shaheen Bagh area with the arrest of three more people on Friday, officials said.
An Indian and two Afghan nationals were arrested on Friday, they added.
The agency took the foreign nationals into custody from Delhi while the Indian man was nabbed from Uttar Pradesh. A 40-year-old man was earlier arrested by the Narcotics Control Bureau (NCB) in the case.
All the accused have been arrested under the provisions of the Narcotic Drugs and Psychotropic Substances (NDPS) Act.
NCB Director General (DG) SN Pradhan said "terror links cannot be ruled out" in this case and the agency is probing these connections further.
The federal anti-narcotics agency had on Thursday said it had seized about 50 kg of "high-quality" heroin packed in Flipkart and other e-commerce companies' wrappers, following raids at Shaheen Bagh-Jamia Nagar on April 27 as part of busting an Indo-Afghan drug-smuggling racket.
NCB Deputy Director General (operations) Sanjay Kumar Singh had said during a press briefing that cash amounting to Rs 30 lakh was also seized after the operation at an apartment in that area.
Mr Singh had said this was one of the biggest narcotics seizures in Delhi in the recent past and the agency had acted upon inputs provided by a "reliable source." Another 47 kg of "suspected" narcotics were also seized from the premises and the NCB has sent the seized substance to a laboratory for testing, Mr Singh said.
The seized heroin originated from Afghanistan and the cash found is suspected to have been channelled through hawala (illegal routing of cash), he added.
The agency also seized cash-counting machines and some other "incriminating" material from the premises.
"It has been revealed that an Indo-Afghan syndicate based in Delhi, the National Capital Region and the neighbouring states is connected with the case. These syndicates have expertise in manufacturing and adulterating heroin locally," Mr Singh said.
A senior officer had said the "kingpin" of the syndicate is based in Dubai and the agency is probing the case further, including certain links with Pakistan-based narcotics operatives.
The India-Afghanistan syndicates smuggle goods into India through maritime as well as land-border routes and heroin is smuggled with legitimate goods and cargo, the NCB said.
The contraband is subsequently extracted from these goods by the Indian members of the syndicates with the help of some Afghan nationals, it added.
The agency is carrying out raids to bust the entire network and the traffickers are linked to operatives based in Punjab, Haryana, Uttar Pradesh, Uttarakhand and Delhi, the NCB had said.
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