Singapore's central bank said it was running a joint investigation with police into possible breaches of securities laws. The authorities also reportedly raided several brokerages.
The Monetary Authority of Singapore (MAS) said as part of its probe, conducted with the commercial affairs department of the Singapore Police Force, it had "obtained documents and items from several broking firms and trading representatives."
The MAS said it wasn't able to provide further information as the investigation was ongoing.
"As a matter of policy, MAS does not comment on its supervisory dealings with specific financial institutions," an MAS spokesperson said via email.
The spokesperson said the MAS statement was made in response to a request for comment on Thursday from local newspaper the Business Times, which on Friday reported that white-collar crime investigators had visited the trading floors of DBS Vickers, Maybank Kim Eng, OCBC Securities and Phillip Securities on Monday to take "certain items" as well as taking some people in for questioning.
OCBC Securities, Maybank Kim Eng and DBS Vickers declined to comment when contacted by CNBC. A representative from Phillip Securities didn't immediately return a phone call seeking comment.
Citing sources, the newspaper reported that the people taken for questioning were remisiers - the name for a licensed trading representative who is attached to a brokerage firm that is an SGX member.
An SGX spokesperson said via email that the exchange cannot comment on the reported investigations, but that it regularly refers case investigations to MAS.
"In 2015, we referred 11 cases of insider trading and 13 cases of market manipulation to MAS. In January to March 2016 quarter, three cases of insider trading and six cases of market manipulation were referred," the spokesperson said.
In 2013, Singapore authorities began investigating market moves after extreme volatility in penny stocks erased billions in market value.
Three penny stocks - Blumont Group, LionGold and Asiasons Capital - surged more than 800 percent over several months before crashing more than 90 percent in October 2013, without an apparent reason. That spurred the Singapore Exchange to impose tighter trading rules.
In January, Bloomberg reported that Lee Boon Ngiap, head of the Monetary Authority of Singapore's capital markets group, said in court documents that the penny-stock crash appeared to have been "an elaborate scheme."
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—By CNBC.Com's Leslie Shaffer; Follow her on Twitter @LeslieShaffer1