WASHINGTON (AP) — Dozens of tea party groups and other conservative organizations of the kind subjected to improper scrutiny by the Internal Revenue Service operated with small budgets and rarely displayed overt partisan activities, according to an Associated Press review of public tax filings by 93 such activist groups. A few groups built million-dollar operations and political ties that could have been legitimate grounds for IRS investigation, tax law experts said.

The AP reviewed 990 tax returns for nonprofit groups that were made publicly available and posted on both the Guidestar and the Foundation Center websites, searching between 2009 and 2011 under the terms "tea party," "patriot" and other terms frequently used by tea party groups. Several tea party groups also made their tax returns available to the AP. The returns detailed revenues and expenses for the groups, as well as other details. Donors' identifies, however, are shielded from disclosure under federal tax code provisions.

Only 21 of the 93 groups reported annual gross receipts higher than $25,000 between 2009 and 2011, according to the AP review. The $25,000 figure is a threshold for the IRS because an organization's financial strength and revenue sources are important factors in determining its tax-exempt status. Nonprofit groups reporting less than $25,000 a year are allowed to file a short-form, postcard tax return instead of a detailed filing — one indication of a low-budget operation.