Islamabad's authorities are investigating why scores of fish started turning up dead in the city's Rawal Lake since monsoon rains began three days earlier in the week.

In an inter-department letter, an official of the department recommended "to arrange some test for determination of the presence of any toxic element in the water [of Rawal Dam]".

However, an official told Associated Press that laboratory testing showed that water from Rawal Lake is "fit for human consumption".

Samples of water from Rawal Lake that were tested were fine for drinking, said Mushtaq Ahmed, a top administrative official in the capital city.

Rawal Lake is used as the main source of water for Rawalpindi, near the capital. Ahmed said Islamabad gets its water supply from the nearby Simly and Khanpur dams.

Police officer Imran Haider had earlier said that samples of water and dead fish from Rawal Lake had been collected and sent for forensic testing after a complaint was received from the capital's Fisheries Department.

An FIR had been registered in the Secretariat Police Station against a “local mafia residing in [the] surrounding of Rawal Lake”, accusing it of dumping poison in the water. Deputy Director Fisheries Mohammad Sadiq Buzdar was the complainant.

In the application, the Fisheries Department stated that the accused were engaged in illegal fishing and boating activities in Rawal Dam, which was reported to the police and also recorded in several FIRs.

It added that police had carried out an operation against this "mafia", during which 20 boats were confiscated and five people were arrested.

The department says that following the operation, "this mafia has been threatening [the] department and contractor [of] [that it would] contaminate and intoxicate the water to endanger the fish and public at large".

An early chemical test report and copies of earlier FIRs have also been attached with the application.

ASI Imran Haider said Saturday that samples of water and dead fish from Rawal Lake have been collected and sent for forensic testing after a complaint was received from the Fisheries Department.

Local residents claim that for the past month, garbage trucks have been dumping waste in the lake during the night. However, authorities suspected that the contamination may have been chemical in nature.

Police and the fisheries department have not yet issued any alert regarding the situation.