US:Hillary Clinton's presidential campaign will run criminal background checks on major fundraisers to ensure that contributions comply with federal laws that limit the amount any individual can give to a candidate.
The decision came as the Clinton campaign said it was returning $850,000 (€615,000) raised by Norman Hsu, who is under investigation for allegedly paying "straw donors" to contribute to Mrs Clinton and other Democrats.
Mr Hsu is in custody in Colorado after it emerged that he skipped a warrant for his arrest 15 years ago on a charge of grand theft in California.
"In light of recent events and allegations that Mr Norman Hsu engaged in an illegal investment scheme, we have decided out of an abundance of caution to return the money he raised for our campaign. An estimated 260 donors this week will receive refunds totaling approximately $850,000 from the campaign," Clinton spokesman Howard Wolfson said.
Mr Wolfson said the campaign would now scrutinise other major fundraisers, known as "bundlers", who bundle together cheques from dozens of individual contributors.
"In any instances where a source of a bundler's income is in question, the campaign will take affirmative steps to verify its origin," he said.
Federal law limits individual campaign contributions to $2,300 for the primary campaign, although contributors can give a further $2,300 for the general election campaign that can only be used if the candidate wins his or her party's nomination.
Contributors must be US citizens or Green Card holders and there are stiff penalties for those who seek to circumvent the rules by paying third parties to contribute on their behalf.
US newspapers have raised questions about some donors of modest means who contributed to the Clinton campaign through Mr Hsu, notably a family that lives in a small home under the flight path from San Francisco International Airport.
The Wall Street Journal reported that, despite holding down modest jobs, the six members of the Paw family were among the top contributors to Mrs Clinton and other Democrats since 2005, according to Federal Election Commission records. The donations from the Paw family often came on the same date, to the same candidate for similar amounts as those of Mr Hsu, who runs a company that employs one of the Paws.
Mr Hsu first described his fugitive status as a misunderstanding but he failed to appear at a court hearing in Redwood City, California, last week and instead boarded an overnight train to Chicago. As the train passed through Colorado, other passengers noticed Mr Hsu slumped against the door of his sleeper carriage, appearing ill and disoriented.
When he was taken to a hospital in Grand Junction, law-enforcement officials were notified that Mr Hsu had been found and he was placed under arrest.
During the first six months of this year, Mrs Clinton's presidential campaign raised $52 million from individual contributors, second to Barack Obama, who raised $58.5 million.