 -- IRWIN, Pa.—Mitt Romney again insisted he will not release additional years of tax returns, arguing he doesn't want to give President Barack Obama more ammunition to attack his campaign.

In an interview with National Review's Robert Costa, Romney said he's already released "hundreds of pages" of tax records, with more records from his 2011 tax return to come. He argued that should be enough.

"In the political environment that exists today, the opposition research of the Obama campaign is looking for anything they can use to distract from the failure of the president to reignite our economy," Romney told Costa. "And I'm simply not enthusiastic about giving them hundreds or thousands of more pages to pick through, distort and lie about."

Asked why he hadn't gotten rid of his offshore investments, which have been the subject of several Obama attack ads, Romney reiterated that he has no influence over the make-up of his investments, which are kept in a blind trust. But he offered new details about at least one of his investments: a Cayman Islands account.

"The so-called offshore accounts in the Cayman Islands, for instance, is an account established by a U.S. firm to allow foreign investors to invest in U.S. enterprises and not be subject to taxes outside of their own jurisdiction. So in many instances, the investments in something of that nature are brought back into the United States," Romney said.

He added: "The world of finance is not as simple as some would have you believe. Sometimes a foreign entity is formed to allow foreign investors to invest in the United States, which may well be the case with the entities that Democrats are describing as foreign accounts."

Romney's explanation is likely to fuel more questions about his overseas investments, given that he is not technically a foreign investor.

Asked about his vice presidential search, Romney offered no details, but he explained what he's looking for.

"In my own view, the people I've worked with over my career have been people who have the capacity to lead, who share my philosophy, and in some cases, people who provide perspectives and skills that I may not share," Romney said.