Or so the Central Intelligence Agencytold President Harry S. Truman. The year was 1951.

It has become conventional wisdom, repeated by President Obama at thenuclear summit meeting this week, that the cold war danger of huge strikes by thousands of nuclear missiles has given way to a new threat: terrorists killing tens of thousands of Americans with a stolen or homemade nuclear device. A broad range of security experts agree that nuclear terrorism may well be the most serious danger the United States faces today.

But it is not new. In fact, almost from the invention of the atomic bomb, government officials were alarmed by the threat that compact nukes would be smuggled into the United States by Soviet agents and detonated.

“Officials regard the possibility of atomic sabotage as the gravest threat of subversion that this country, with its virtually unpatrolled borders, has ever faced,” The New York Times reported in 1953, telling readers that the Eisenhower administration was preparing to alert the public to the danger from “valise bombs.”

Hundreds of pages of declassified documents from the 1950s, obtained by The New York Times from the F.B.I.under the Freedom of Information Act, lay out a strikingly familiar story, in which Communist agents played the role of today’s Al Qaeda.

Then, as now, investigators searched for agents they feared were in the United States awaiting orders to attack. Then, too, the government spent millions to install radiation detectors at airports and seaports despite doubts about their effectiveness. (In those days, false nuclear alarms were set off by radium watch dials, once hidden in a woman’s corset.)

Nor is the worry in recent years about nuclear material crossing the permeable Mexican border new. An F.B.I. memo from 1953 warned that “a saboteur could easily pose as a Mexican ‘wetback’ and get into the country without detection, presumably carrying an atomic weapon in his luggage.”

Micah Zenko, a fellow at the Council on Foreign Relations who has written on nuclear history, said: “The fear of a clandestine nuclear attack on American soil goes back to the very beginning of the nuclear era. There’s certainly nothing new here, even if they didn’t call it terrorism back in the ’50s.”

Even before the Soviet Union detonated its first atomic bomb in 1949, security officials studied the threat from smuggled weapons. But secret reports concluded that the Soviet Union was likely to try such an attack only as a prelude to total war. An all-out attack, American experts believed, might even begin with the detonation of nuclear weapons smuggled into Soviet diplomatic offices in New York and Washington.

Intelligence officials feared that bomb parts might be delivered in diplomatic mail pouches, carried by international air travelers in their luggage or delivered by boat or submarine to an isolated beach.

Communist agents already in the country might then assemble, plant and detonate the weapons. “Surveillance of all Communist Party members and sympathizers is impossible and impractical since numerically they exceed by many times the total Special Agent force of the F.B.I.,” a bureau memo complained. J. Edgar Hoover, the F.B.I. director, who was intensely focused on the smuggling threat, proposed increasing manpower to cope.

Among many potential nuclear saboteurs, F.B.I. field offices identified the proprietor of a left-wing bookstore in Seattle, a reporter for the Soviet news agency Tass and even a representative of the American Council for a Democratic Greece.

When the Polish consul to Detroit arrived in the United States in the mid-1950s with four big boxes, F.B.I. agents surreptitiously searched them for nuclear material. They found 24 bottles of cherry cordial but “no article or part thereof that could be construed as a portion of a weapon of mass destruction,” their secret report solemnly declared.

The press, too, got in on the act. In 1954, a reporter for The Los Angeles Mirror, a tabloid newspaper, wrote a splashy story headlined “I Smuggled Mock A-Bombs into L.A.,” accompanied by a diagram of a man carrying a “baby A-bomb” in a suitcase.

The smuggling fears began to fade in the late 1950s with the advent of intercontinental ballistic missiles, which posed an incomparably greater threat of surprise attack. But in the half-century that followed, the worry never entirely went away.

Security officials later speculated about whether China might set off a smuggled nuke in the United States and make it look like a Soviet attack, provoking devastating war between its rivals. Later, as portable tactical nuclear weapons proliferated in both Eastern and Western Europe, there were periodic alarms about their security.

After the murder of Israeli athletes by Palestinian agents at the 1972 Olympics in Munich, American officials shifted their focus to terrorists. Their concern increased immeasurably after the Sept. 11 attacks in 2001, when reports that Al Qaeda had actively sought a nuclear weapon since the early 1990s took on a chilling significance.

In the 1950s the United States knew its adversaries had weapons; the mystery was whether they might use them. Today, said Jeffrey T. Richelson, a historian of nuclear weapons, the situation is reversed: Qaeda leaders have suggested publicly that they would use a nuclear weapon, “but as far as we know, Al Qaeda hasn’t even come close to building a bomb.”

Most security experts believe the focus in recent years on destroying or locking up nuclear material is far more effective than sealing American borders. The global effort to reduce the threat was advanced at the summit meeting in Washington this week, with commitments from many countries to destroy or secure supplies of plutonium and highly enriched uranium.

Knowing the history of periodic panics about smuggled nukes offers a kind of reassurance in the face of a horrifying danger, said Mr. Zenko of the Council on Foreign Relations.

“If you consider that the threat has been around for more than 60 years,” he said, “you don’t get overwhelmed by fear.”