Computer scientists from California universities have hacked into three electronic voting systems used in California and elsewhere in the nation and found several ways in which vote totals could potentially be altered, according to reports released yesterday by the state.

The reports, the latest to raise questions about electronic voting machines, came to light on a day when House leaders announced in Washington that they had reached an agreement on measures to revamp voting systems and increase their security.

The House bill would require every state to use paper records that would let voters verify that their ballots had been correctly cast and that would be available for recounts.

The House majority leader, Representative Steny H. Hoyer, Democrat of Maryland, and the original sponsor of the bill, Representative Rush D. Holt, Democrat of New Jersey, said it would require hundreds of counties with paperless machines to install backup paper trails by the presidential election next year while giving most states until 2012 to upgrade their machines further.