Washington ? Enron Corp. has agreed to release its tax returns since 1985, and those of its affiliates and partnerships, in a Senate Finance Committee investigation into whether the bankrupt company improperly avoided paying taxes.

In a letter to senior Finance Committee members, attorney Fred Goldberg said Enron will cooperate by waiving federal tax privacy protections so that Congress can determine whether laws need to be changed in the aftermath of the energy trading company’s downfall.

Sen. Max Baucus, D-Mont., the Finance Committee chairman, said Friday the panel has begun a “thorough and comprehensive probe” into the use of tax shelters or other devices that might have played a role in Enron’s demise.

The question, Baucus said in a statement, is “whether Enron may have engaged in aggressive tax planning to improperly avoid paying federal income taxes or exploited loopholes in our tax system requiring attention.”

In another congressional inquiry, Sens. Joseph Lieberman, D-Conn., and Fred Thompson, R-Tenn., said they were sending letters to four federal agencies and issuing subpoenas to Enron and its former auditor, Arthur Andersen LLP, seeking information on company contacts with the agencies.

The White House disclosed last month that former Enron chairman Kenneth Lay had been in touch with President Bush’s treasury and commerce secretaries and the White House budget director as the company foundered in October and November.

Lieberman is chairman and Thompson the senior Republican of the Senate Governmental Affairs Committee. Its investigative subcommittee issued 51 subpoenas last month to officials of Enron and Andersen.

The new subpoena to Enron seeks, along with contacts between the company and the Labor Department, the Securities and Exchange Commission, the Federal Energy Regulatory Commission and the Commodity Futures Trading Commission, information about bonuses, deferred compensation and severance payments for Enron executives. Deferred compensation was set aside for later payment in accounts tied to the value of Enron stock.

Knowing Enron was about to file for bankruptcy, executives quickly withdrew millions of dollars in compensation and bonuses in late November that they had deferred. Money was wired to some bank accounts in less than 24 hours.

Through use of tax deductions for stock options, entities in offshore tax havens and tax shelters, Enron appears to have avoided paying corporate income taxes in four of the past five years. The bookkeeping netted a total refund of $381 million, according to an analysis by the labor-funded Citizens for Tax Justice.

On the other hand, Enron had accumulated more than $250 million in credits for alternative minimum taxes, indicating it had paid those in the past.