"A party can be caught up in an investigation without having done anything wrong," she said. "Brown & Brown was never accused of anything."

The insurer was concerned that confidential information would be shared in an "unlimited way," she said. "We had no idea who it was disclosed to."

Blumenthal, the Democratic candidate for U.S. Senate, said in a statement that the decision confirmed his "broad power" to investigate violations of Connecticut's antitrust law. However, he acknowledged that the ruling imposes "some parameters" on his ability to share documents outside his office.

Blumenthal sought information that contained trade secrets and other valuable commercial and financial information, the Supreme Court said.

Blumenthal and Brown & Brown disagreed over provisions of Connecticut's antitrust law, which allows the attorney general to cooperate with federal and other state officials in sharing and disclosing information. The law also permits disclosures at the attorney general's discretion.

But the Supreme Court said the state's trade secrets law allows a court to halt and punish the appropriation of trade secrets without consent, and said that even documents provided under the antitrust law may not be made available to the public by the attorney general.

If Blumenthal wants to share materials and information with other government officials, he must obtain agreement from those officials that they will abide by the same confidentiality restrictions that apply to him, the court said.

The justices also said they "strongly reject" an argument by Blumenthal that little harm would result from disclosing subpoenaed materials to competitors of a business being investigated because competitors "will very likely already have some knowledge of the document as a result of" participating in a contract or conspiracy being investigated.

The Supreme Court said targets of an investigation by the attorney general "have not been, and in some cases may never be, charged with any wrongdoing."

"Accordingly, it is wholly inappropriate to presume their guilt to justify further expansion of the defendant's already considerable statutorily granted investigatory powers," the justices wrote.

A spokeswoman for Blumenthal said the investigation is continuing and has recovered about $1 billion for businesses, consumers and others in the last few years.