Effect of Libor on US loans examined US lawmakers have raised concerns that the alleged manipulation of the London Interbank Offered Rate, or Libor, may have harmed households, raising the stakes on a scandal that thus far has been confined to Wall Street and the City of London. There are at least 900,000 outstanding US home loans indexed to Libor that were originated from 2005 to 2009, the period the key lending gauge may have been rigged, investigators have said. Those mortgages carry an unpaid principal balance of $275bn, according to the Office of the Comptroller of the Currency, a bank regulator. During periods when banks were allegedly attempting to push Libor higher, households with loans tied to the gauge may have paid higher rates than necessary. However, if the rate was manipulated lower, households may have benefited from paying below-market interest rates. "I think the US government should be just as aggressive in getting to the bottom of this scandal as the United Kingdom has been," said Senator Sherrod Brown, chair of the bank regulatory subcommittee on the Senate banking committee. "This was not isolated to London, but affected tens of millions of investors, borrowers and taxpayers in our country as well," Mr Brown added. Senator Jack Reed, a fellow Senate banking committee member, said that US policy makers do not know the extent of the possible harm on households. We won't know until there is a very thorough and exhaustive analysis. That should be done immediately on a broad-based level by every regulator with jurisdiction," Mr Reed said. The number of US home loans in the OCC study represents 3 per cent of mortgages originated from 2005 to 2009. Investors have already filed lawsuits against banks for alleged Libor manipulation, court filings show. Class-action suits representing numerous households could be next, lawyers said. On June 27, Barclays agreed to pay US and UK authorities $450m to settle allegations it had attempted to manipulate Libor. An ongoing global investigation threatens as many as 20 banks and inter-broker dealers that may have been involved in manipulating an interbank rate that sets the terms for hundreds of trillions of dollars of financial instruments. Senator Mark Warner, a member of the Senate banking committee, said: "Banks manipulating Libor is an enormous issue that not only represents a fraud on bank customers but has an impact on smaller borrowers and lenders around the world and undermines the entire credibility of financial markets."