House rules prohibit lawmakers and staff from taking trips paid for by registered lobbyists or lobbying firms.
David G. Lopez was Doolittle's chief of staff in July 2001 when he spent a week in Puerto Rico on what he described on a travel disclosure form as a "fact finding" trip.
The form lists the sponsor that paid for the trip as Greenberg Traurig, the lobbying firm where Abramoff worked before pleading guilty to felony charges last month in a wide-ranging political corruption investigation.
Lopez left Doolittle's staff a year ago to spend more time with his family in California.
"Although this incident took place nearly five years ago, I specifically remember consulting with the Ethics Committee several times, and my clear intention was to live within the rules, especially since I properly disclosed this trip in a timely fashion," he said in a statement issued after being contacted by Doolittle's press secretary.
"My recollection was that the firm was to be reimbursed by the sponsor, which I thought was consistent with the rules. I don't recall that Congressman Doolittle knew the specifics of who paid for the trip, and if I inadvertently made a mistake I will take every necessary step to correct it."
Lopez's statement did not say what he did in Puerto Rico or who he thought would reimburse Greenberg Traurig.
Ethics Committee guidelines say that the prohibition against accepting lobbyist-paid travel applies even when the lobbyist will be reimbursed by a non-lobbying client.
The trip is the latest example of connections between Abramoff's interests and Doolittle, a conservative ally of former House Majority Leader Tom DeLay, R-Texas, and a member of the House GOP leadership. Those ties have caught the attention of federal investigators.
Doolittle accepted at least $14,000 in campaign money from Abramoff and tens of thousands of dollars more from Abramoff's tribal clients, and wrote letters to the Interior Department on behalf of two tribes that were clients of Abramoff's firm.
Doolittle's wife did fundraising work for Abramoff, and another former Doolittle aide, Kevin Ring, later went to work for Abramoff as a lobbyist.
The travel disclosure form, which is required by House rules, shows Lopez was in Puerto Rico from July 7-14, 2001. It lists transportation costs for the trip as $1,352 and says he spent July 8 and July 12-13 there at his personal expense. Doolittle signed the form.
Before joining Greenberg Traurig in January 2001, Abramoff represented a Puerto Rican pro-business group while a lobbyist at another firm, Preston Gates Ellis & Rouvelas Meeds. Ring also was registered to lobby for the group, Future of Puerto Rico, Inc.