Investigators probing the murder of West Bengal CM Suvendu Adhikari's close aide Chandranath Rath have found a key lead in the form of a UPI transaction made at a toll plaza in Howrah, an officer said on Sunday.

The silver car allegedly used in the killing crossed the Bally toll plaza shortly before the attack, and the occupants paid the toll through UPI, he said.

"The UPI transaction helped the investigators identify a mobile number linked to one of the suspects. CCTV footage from the toll plaza also captured images of the vehicle and its occupants," he added.

Rath was shot dead in Madhyamgram's Doharia Lane, around 7 km from Kolkata airport, on the night of May 6, in what BJP leaders described as a "planned assassination".

The murder, which happened two days after the BJP won a landslide mandate in the assembly elections, bringing down the Mamata Banerjee-led government of 15 years, triggered a furore in the state.

Rath's SUV was intercepted by the silver car near his residence, following which armed men on two-wheelers opened fire at point-blank range, according to police.

After the attack, the assailants abandoned the silver car and fled in a red car and a motorcycle. Two motorcycles linked to the crime were later found, one near the spot and another in Barasat, around 6 km away.

The registration plates of all the vehicles were allegedly forged, and their engine and chassis numbers were tampered with, police said.

The CID and the STF have jointly formed a Special Investigation Team (SIT) to probe the case.

Investigators have also identified the weapon allegedly used in the murder as an Austrian-made Glock 47X pistol, a sophisticated firearm whose use has rung alarm bells among both state and central intelligence agencies.

"A Glock pistol is not easily available in India. Its use prompted investigators to examine whether there could be any international or cross-border link behind the crime," the officer said.

Agencies are also examining whether any foreign hostile network could have used Bangladesh as a transit route, he said.

"We are keeping all possibilities open, including the involvement of external operatives or organised criminal networks," the officer added.

Investigators believe the killing bore the hallmarks of a professionally executed contract murder.

An officer associated with the probe said the attackers appeared to have conducted extensive reconnaissance before carrying out the operation.

"The spot chosen for the shooting was outside CCTV coverage, and the timing suggested careful planning," he said.

Investigators suspect the assailants may have been hired shooters from Uttar Pradesh or from across the Bangladesh border, which is around 65 km away from the spot of the crime, he added.

Central agencies are also examining possible links between the weapon used in the murder and recent recoveries of sophisticated foreign-made firearms along the India-Pakistan border, one of the officers quoted earlier said.

Madhyamgram in North 24 Parganas district has previously figured in cases involving fake passport rackets and cross-border movement, making the area sensitive from an intelligence perspective, officials said.

A state police officer said all possible angles, including personal enmity and criminal motives, are being examined.

Another officer said Rath had access to several sensitive files linked to alleged cattle smuggling and coal mafia networks, and investigators are looking into whether the murder was aimed at destroying or gaining access to evidence.

"No theory has been ruled out at this stage. The investigation is progressing on multiple fronts," he said.

No arrests have been made so far in connection with the murder.