Top-ranking Republicans on Wednesday escalated their inquiry into a controversial FBI agreement to destroy the laptops of two Hillary Clinton aides questioned in the email scandal investigation, pressing Attorney General Loretta Lynch for answers and suggesting the deal obstructed congressional investigators.

The agreement to destroy the computers, the lawmakers wrote Wednesday in a letter to Lynch, “is simply astonishing given the likelihood that evidence on the laptops would be of interest to congressional investigators.”

The letter, signed by four Republican committee chairmen, follows allegations Monday that immunity deals for former Clinton chief of staff Cheryl Mills and ex-campaign staffer Heather Samuelson included a side arrangement obliging the FBI to destroy their laptops after reviewing the devices.

Sources said the arrangement also limited the search to no later than Jan. 31, 2015. This meant investigators could not review documents for the key period after Clinton's use of a private email and server as secretary of state became public.

House Judiciary Committee Chairman Bob Goodlatte, R-Va., first asked Lynch about the terms in a letter Monday and co-signed the latest letter Wednesday along with House Oversight Committee Chairman Jason Chaffetz, R-Utah; House Intelligence Committee Chairman Devin Nunes, R-Calif; and Senate Judiciary Committee Chairman Charles Grassley, R-Iowa.

The lawmakers asked specifically about letters from attorney for the aides Beth Wilkinson that described the restrictive terms -- and were recently reviewed by committee staff. The Republicans expressed “concern” that the “FBI inexplicably agreed to destroy the laptops knowing that the contents were the subject of Congressional subpoenas and preservation letters.”

The letter repeatedly cited Congress’ interest in the “evidence” that may have been jeopardized under the side arrangement.

The new letter asked Lynch why the FBI agreed to destroy the laptops and, significantly, what legal authority the FBI has to destroy records subject to a congressional investigation or subpoena. The letter also asked if the FBI followed through and in fact destroyed “evidence” from the laptops or the laptops themselves. 

Asked for comment, a Justice Department spokesman said: "We have received the letter and are reviewing it."

The side deals were agreed to on June 10, less than a month before FBI Director James Comey announced the agency would recommend no charges be brought against Clinton or her staff.

Judiciary Committee aides told FoxNews.com that the destruction of the laptops is particularly troubling as it means that the computers could not be used as evidence in future legal proceedings, should new information or circumstances arise.

Fox News’ Catherine Herridge and FoxNews.com’s Judson Berger and Adam Shaw contributed to this report.