New Delhi: Sleuths of the National Investigation Agency (NIA) investigating the Burdwan blast have expressed concerns over their security, sources said on Thursday.

Source also added that the Centre has taken cognizance of their concerns and decided to send one company of the Border Security Force (BSF) to provide cover to the NIA investigators.

NIA teams set up camps in Burdwan, Kolkata, Nadia and some parts of Assam in connection with the investigation.

The BSF has been on high alert since the Burdwan blast. A high-alert was sounded across West Bengal while its borders with neighboring countries and adjacent states were sealed after intelligence sources revealed that a video of al Qaeda leader Ayman al-Zahawari was being circulated in Bangladesh.

The North Bengal Border Security Forces (BSF) tightened security system on 199 border outposts (BOPs) including Fulbari at the Jalpaiguri.

The NIA also had to quell rumours that the West Bengal Police and CID were not cooperating in the investigations.

Amid allegations that the state police was destroying evidence to cover up, the union home ministry had directed the NIA to take up the investigation into the incident in which the Jamaat-ul-Mujahideen Bangladesh is suspected to be involved.

The opposition Communist Party of India-Marxist and the Bharatiya Janata Party had demanded an NIA probe alleging that the ruling Trinamool Congress was indulging in a cover up to hide its "terror links". The Mamata Banerjee government had opposed the central agency probe contending it to be an attack on federalism.

Six people including two women have been arrested for the blast that occurred in a house in Khagragarh area in which a militant was also injured.

The NIA conducted searches at various places including the rented residence of killed militant Shakil Ahmed in Beldanga in Murshidabad district and recovered incriminating documents and metal pieces used in making IEDs. The NIA team and Bengal CID also conducted a thorough search of the Simulia Madrassa.

Investigations so far have revealed that the two women arrested by NIA custody were radicalised in violent Jihadi ideology at the Simulia madrassa.

Besides the six arrests in Bengal, six others have been nabbed from Barpeta district in Assam.