Seven small-cap companies listed in Singapore have been asked by the police to assist with investigations into potential violations of the nation's securities laws, after three commodity firms together lost over $6bn in market value last October.

Telecommunications investment firm Innopac Holdings, diversified group IPCO International, ISR Capital and electrical-appliances wholesaler ITE Electric confirmed that they had received police requests, seeking access to electronic data and hardware used by senior officials and employees.

Similar disclosures were made on 2 April by three other firms: Blumont Group, LionGold and Magnus Energy Group.

The requests came as Singapore police initiated a criminal investigation into possible trading irregularities in the shares of commodity companies Asiasons Capital, Blumont and LionGold, whose stocks tanked in October 2013.

Singapore's police force and the nation's central bank said on 2 April they were jointly investigating suspected trading irregularities in the three commodity firms – they are under investigation for potential breaches of the Securities and Futures Act, according to a statement on the website of the Monetary Authority of Singapore.

Many of the aforementioned companies have invested in each other, reported the Wall Street Journal.

Asiasons is LionGold's second-largest shareholder with an 8.9% stake, and it also owns more than 25% of ISR Capital, according to company filings.

IPCO owns 9.74% of Blumont and 7.2% of Innopac, according to corporate filings.

Blumont and LionGold both hold stakes in Innopac, at 5.06% and 3.43% respectively.

ITE Electric's independent director Goh Hin Calm owns 3.85% of Innopac, states Innopac's latest annual report.

Magnus owns a 1.99% stake in LionGold, states LionGold's latest annual report. LionGold Executive Director Wira Dani Abdul Daim is a significant shareholder in both Magnus and ISR Capital, according to company annual reports.

Blumont has a 1.35% stake in ITE Electric, according to corporate records.

Trading in Asiasons, Blumont and LionGold came under scrutiny in October 2013, when the firms lost over 8bn Singapore dollars ($6.33bn, £3.81, €4.62bn) in combined market value in just two days.