Rep. Luis Gutierrez (D-Ill.) (The Washington Post)

Updated and corrected 5:16 p.m.

There is "substantial reason" to believe that Rep. Luis Gutierrez (D-Ill.) violated the rules of the House of Representatives and possibly federal law by paying his former chief of staff at least $590,000 in fees over 10 years while the former aide was also working as a lobbyist, according to a congressional investigation released Monday.

In response, the House Ethics Committee announced that it plans to continue reviewing the allegations against Gutierrez but will not empanel a special investigative subcommittee, which would be an indication of a more serious investigation. The committee released a report detailing allegations that Gutierrez improperly used taxpayer dollars to pay his former chief of staff, Doug Scofield, for "training" and "non-legislative" services over a decade while Scofield was working as a Chicago-based lobbyist.

The allegations are detailed in a 177-page report that was prepared by the Office of Congressional Ethics, which investigates allegations of wrongdoing against lawmakers on behalf of the ethics committee.

"As the Committee reviews this matter, Congressman Gutiérrez and his office will continue to cooperate fully," Gutierrez's office said in a statement Monday. The lawmaker cooperated fully with the investigation and provided 10 years of notes and e-mails between Scofield, Gutierrez and the lawmaker's staff, the statement said.

Members of the OCE voted unanimously late last year to refer the matter to the full Ethics Committee, finding that Gutierrez had paid Scofield about $590,000 since 2003, most of it since 2008. Under an agreement with Gutierrez, Scoffield was hired to provide "staff development and training" and other "non-legislative, general office services" at a cost of $4,500 to $6,000 per month. But the OCE report found that Scofield's work likely "exceeded the permissible services" that people who are not official House employees can provide to congressional offices.

Under official House rules, members may use their official budgets only for "official and representational expenses." The money cannot be used to pay for "activities or events that are primarily social in nature, personal expenses, campaign or political expenses," according to the rules.

Gutierrez is a longtime lawmaker from Chicago and a strong supporter of enacting new immigration laws. His office previously said that Gutierrez had cancelled his contract with Scofield last year.

Separate from the Gutierrez matter, the Ethics Committee has other pending investigations against Reps. Cathy McMorris Rodgers (R-Wash.) and Markwayne Mullin (R-Okla.), among others.

CORRECTION: Scofield is based in Chicago, not Washington.