No sooner had Gov. Gavin Newsom cut billions of dollars in spending to close a budget deficit in June than California received an unexpected tax windfall, one that has people in the Capitol speculating about where the avalanche of money came from.

More corporate taxes than expected poured into state coffers this summer, with cash receipts exceeding forecasts by nearly $2 billion since April. An especially big surge came in July, and state officials and accounting experts think the extra receipts came from a small number of companies -- most likely one or more Silicon Valley tech firms, with artificial intelligence chipmaker Nvidia a leading candidate.

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This July, the Finance Department said it collected about $1.4 billion in corporate taxes, nearly three times the agency's forecast of $500 million. In June, corporate taxes were $263 million above forecast, and in May, $752 million over. "The July overage was likely due to large payments by a small number of companies and may not necessarily be indicative of overall corporation tax revenue trends," the department said in its monthly bulletin.

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But the July influx in corporate tax payments was likely related to changes in state tax rules adopted in June, according to state and accounting experts who spoke with CalMatters. The tax changes, intended to help close the deficit, include a suspension of a deduction businesses can claim to offset profit, called the net operating loss deduction, as well as a $5 million limit on how much businesses can claim for research and development and other tax credits.