In this file photo from Tuesday, Dec. 20, 2016, Ahmad Khan Rahimi, the man since convicted of setting off bombs in New Jersey and New York in September is led into court in Elizabeth, N.J. (Photo: AP/Mel Evans, File)

The two major U.S. terrorist attacks of the 1990s, at the Murrah Federal Building in Oklahoma City in 1995 and the World Trade Center in 1993, both involved thousands of pounds of precursor chemicals packed into trucks parked outside the targets.

The new millennium’s terrorist bombings, abroad in Paris, Brussels and Manchester, and domestically last year in New York and New Jersey, show that even though smaller amounts of the substances were used, improvised explosive devices (IEDs) can still be made by chemicals available to private consumers through retail outlets.

A new report by the National Academy of Sciences says more stringent regulations and oversight need to be imposed on the makings of such bombs—even if the threat can never be fully eliminated because of the sheer number of chemicals and recipes out there.

“Twenty years after the Oklahoma City bombing, the ingredients, components, and instructions for producing IEDs remain accessible to terrorists, violent extremists, and criminals … who are intent on inflicting casualties, damaging critical infrastructure, and eliciting fear,” the report states.

“From the outset, we recognized that as long as explosive materials such as black and smokeless powders are readily available, the threat of IED attacks cannot be eliminated,” added Victoria Greenfield, chairwoman of the 13-member committee that wrote the report. “Nevertheless, we identified a set of possible control strategies, featuring different types of restrictions on access to precursor chemicals that could play a part in risk reduction.”

The group focused on the precursor chemicals that have proven to be used in bombs over the last few decades, from the 1990s truck explosives, to the devices carried by killers like the pressure cookers used in the 2013 Boston Marathon attack, and the pipe bombs employed in the three detonations in New York and New Jersey in September 2016.

The group assessed three categories of substances, broken down into A, B and C lists of items that can be purchased at hardware, drug and garden-supply stores. The A list includes the highest priority chemicals—including the ammonium nitrate that Timothy McVeigh used in the Oklahoma City attack in 1995 and the urea nitrate Ramzi Yousef and Eyad Ismoil used to concoct the bomb for the 1993 World Trade Center bombing. Other priority chemicals include nitric acid, potassium chlorate, potassium perchlorate, sodium chlorate, nitric acid and even hydrogen peroxide, among others.

Although many of the highest-priority chemicals are tightly controlled, the internet’s databases of know-how and instructions has opened up the possibilities for home-made bombs, the report adds. The instructional videos include how to make homemade explosives such as triacetone triperoxide to fuel a main charge within an IED with shrapnel, boosters, initiators, switches and a power source.

The group came up with four potential solutions, three of which included controls such as bans or licensing strategies. But they stopped short of endorsing any of the four options, since “the committee lacked the time, resources, and directive from (the Department of Homeland Security) to conduct a comprehensive and detailed analysis of policy options.”

“(A full) analysis should also consider the results of existing domestic programs that restrict access to precursor chemicals, including those intended to curb illicit drug production, and programs adopted in other countries,” they add.

“This report considers the benefits, costs, and uncertainties of each approach, but does not provide the comprehensive analysis of specific proposals that would be necessary for policy making,” added Greenfield, also a visiting scholar at George Mason University’s Department of Criminology, Law and Society.

The newest report echoes some of the findings from a 1998 National Research Council report, which resulted in the tighter control of some of the key substances used in Oklahoma City and at the World Trade Center.