Ability to Prevent Attacks in U.S. Is Limited : Violence: Weapons, materials for bombs are readily available. Immigration policy makes it difficult to keep extremists out, experts say.

The man arrested Thursday in the World Trade Center bombing appeared to have made remarkably crude attempts to cover his tracks. Yet the same person allegedly helped create and detonate a powerful explosive without detection by authorities.

As that irony demonstrates, a huge bomb can be created and planted in a public facility by relatively unsophisticated people, counterterrorism officials said Thursday.

At a press conference, acting Atty. Gen. Stuart M. Gerson said there was no indication that law enforcement missed any opportunity to prevent the deadly attack last Friday near the heart of the U.S. financial center, which killed five and injured more than 1,000.

“There’s nothing that suggests that anyone missed a beat on this, or that any information was not shared,” Gerson said. “I haven’t heard anything that suggests any lapse on the part of any agency, any tip unheeded, any information unshared. That’s not what characterizes this.

“We live in an open society where it’s difficult to monitor the activities of all people here at all times,” Gerson said. “We don’t try to do that. It’s a question of national purpose.”

FBI Director William S. Sessions, appearing with Gerson at the same press conference, was a bit more positive about the ability of U.S authorities to prevent terrorist activity. “We might conceivably have prevented this thing, if it turns out to be a terrorist activity,” Sessions said.

In the five years from 1987 to 1991, the FBI thwarted 24 terrorist attempts in the United States, according to FBI records.

Even so, officials noted, the country’s very strengths--its political system, its tolerance of extremist views and its historical embrace of immigrants--will continue to make Americans vulnerable to terrorism.

“This is a democratic country and one that is not all that difficult to get into,” a senior counterterrorism official said. “We try to be open to the various and sundry oppressed peoples of the world. Obviously we want to keep out the bad guys but you have to have a reason to do it.

“And absent some pretty good reasons, there’s little you can do to stop the flow. This is, after all, a country of immigrants.”

And historically, immigrants from around the world have brought with them a wide assortment of ideological and religious beliefs.

“There have always been radical emigres living in this country,” said Bruce Hoffman, a terrorism specialist at the RAND Corp. in Santa Monica.

Added to those elements is the relative ease of acquiring the ingredients and knowledge for making terrorist devices in the United States.

State laws on explosives vary, with some states having virtually no regulations on purchase, use or quantity of explosives that can be sold, nor requirements for any form of licensing. Most explosives are available because of their wide use in mining and construction work.

As a result, Americans or foreigners can purchase large quantities of explosives over a short time--rather than having to rely on smuggling foreign shipments into the United States.

Statistics bear out the effects of such open trade in explosives. In 1991, more than 2,000 bombings at one level or another were reported to the FBI, according to U.S. counterterrorism officials. That represented a 58% increase from the year before. Most of the bombs were made from commercially available explosives, the sources said.

Extremists interested in building bombs do not have to look far for instructions. They are available in technical and general books and a wide range of other sources.

Terrorists also have easy access to other types of weaponry through thousands of commercial gun dealers. The Washington Post reported Thursday that 1.9 million Chinese weapons were sold in the United States between 1989 and 1991, including AK-47 semiautomatic assault guns made by the Chinese defense industry. A gunman used an AK-47 to kill two people and wound three others as they sat in their cars in turn lanes near CIA headquarters in suburban Washington on Jan. 25.

Other, even more lethal military weapons are readily available through sources that commonly advertise in magazines popular among paramilitary enthusiasts.