BRUNSWICK — Former Maine governor and independent U.S. Senate candidate Angus King released seven years of his federal tax returns Monday.

In eight separate messages e-mailed to reporters King’s campaign staff released the jointly filed income tax returns of King and his wife, Mary Herman.

The couple’s taxable income in 2011 was just over $324,000. Their biggest taxable income year, in the documents released, was 2009 when they paid taxes on $474,000.

The couple’s gross income for the seven years averaged $568,868 a year, the release stated.

The couple’s effective tax rate for the seven years ranges from 20 percent to 26 percent, according to a press release issued with the documents.

The couple also contributed 13.4 percent of their income to charity with most (97 percent) of those donations going to Maine-based organizations, the release stated.

Those annual donations ranged from about $39,000 in 2006 to more than $100,000 in 2009.

King’s campaign was following up on a challenge from the campaign of the Democratic candidate Cynthia Dill, a state senator from Cape Elizabeth.

“This is unprecedented in Maine history,” Crystal Canney, a spokeswoman for King’s campaign said.

She said King was also the only candidate to file his federal Senate Financial Disclosures, which look back at the last 18 months of the couple’s financial dealings, on time.

The release states the bulk of King’s wealth came from the sale of his “energy conservation” business just prior to his entrance in the race to replace retiring U.S. Sen. Olympia Snowe, R-Maine.

Dill said all three leading candidates should release 10 years of tax returns. Her campaign called for the release of the tax information twice in September, the second time singling out King.

In those releases Dill said Republican candidate and Maine Secretary of State Charlie Summers had agreed to release his tax returns as well. Last week she asked King to answer her by 5 p.m. Monday.  Instead King’s campaign simply released his tax returns to the media.

On Monday, Dill thanked King for joining her and Summers in agreeing to release the income tax information.

“I am prepared to release 10 years of tax returns and am glad that the other two candidates have risen to my challenge to be transparent with Maine voters,” Dill said in a written statement. “In Maine, we have seen millions of dollars being spent by the other two U.S. Senate candidates and their acolytes on deceptive media advertising. Against that backdrop, and in the spirit of public accountability, I welcome the fact that our campaign has again shined a light on issues of importance to Maine voters.”

Canney said King’s accountant keeps only seven years on file. She also said they felt it was important to release all the tax data they had available in an effort at “full transparency.”

Summer’s campaign spokesman, Drew Brandewie, also issued a statement Monday saying they had contacted Dill’s campaign “in an effort to work together in a bipartisan way to comply with her proposal.”

“We’re going to post eight years of Charlie’s tax returns on our website tonight,” Brandewie said. “Like a lot of people, Charlie does his own taxes and that’s what he was able to find. We’re going to continue to look and if we find more we’ll release those too.”

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