The dusk-to-dawn curfew on Dade County's riot-torn black neighborhoods was lifted today as National Guard troops began pulling out of the area.

"We are almost back to normal," said public service director Bobby Jones. "It's almost like any other Wednesday in Dade County."

As tensions cooled, federal officials turned their attention to the smoldering resentments that touched off the violence last Saturday.

A federal grand jury was convened this afternoon to investigate the killing of black insurance salesman Arthur McDuffie and the acquittal last week of four white ex-policemen, which triggered the explosion.

One of the first witnesses was Eula Bell McDuffie, the slain man's mother. Holding on to the arm of her attorney, Mrs. McDuffie, wearing a beige print dress, was escorted to the grand jury room by at least nine federal marshals, who wore black armbands for identification. "I hope justice will be done," she said.

Justice Department and FBI officials also promised a stepped-up review of at least six other cases of alleged police brutality that have been pending for months, but the McDuffie case was the only one deemed ready for grand jury consideration. Even in that case, officials said, it will take "weeks, not days" to determine whether any civil rights indictments should be returned.

In an interview, Dade County prosecutor Janet Reno, whose office is caught in a crosscurrent of criticism from black leaders, the Justice Department and a governor's commission, shrugged, bit her nails, and said, "Go ahead, investigate us, investigate us." Although she decried last Saturday's verdict as "an outrageous tragedy," she regards herself as a candidate for scapegoat, with black leaders placing her removal from her elective office as a top demand.

Meanwhile, in Washington, the White House announced that President Carter will visit Miami "in the near future" to emphasize the necessity of rebuilding the scarred Liberty City area.

FBI officials confirmed today they had received an anonymous tip that the four former metropolitan police officers acquitted last week and two others involved in the case had been targeted for assassination.

Lawyers for several of the ex-officers said they had advised their clients to take special precautions after Metro police authorities informed them Monday that the department was too strapped by the rioting to afford any protection to them. Since then, however, the department reportedly has taken measures to guard the men and their families.

"The caller said he overheard a conversation about people coming here from out of town "to assist in a committee for justice" and kill all six officers, said FBI agent-in-charge Arthur Nehrbass. "We couldn't dismiss it. The guy was a rational, well-spoken individual."

Some 700 of the 3,600 National Guardsmen were sent home today while the rest remained on standby in local armories and other make-shift quarters. There were 14-riot related killings in the violence that began Saturday night. Property damage estimates ran as high as $100 million.

While federal authorities began concentrating on charges of police misconduct that have accumulated over the past 18 months, local authorities were pressing their investigations of the most brutal killings that occurred in the midst of the rioting.

Miami homicide Sgt. Mike Gonzalez indicated his department hoped for a breakthrough in the deaths of Bennie Higdon, a 21-year-old baker, and two 15-year-old companions who were driving home from a fishing trip when a mob of hundreds of blacks pulled them from their car. The three whites were pummeled with rocks, cement blocks and a newspaper rack -- and then run over by a green Chevrolet Impala.

"Already we have got some tips on who the offenders are," Gonzalez told reporters.he said getting witnesses might prove difficult, but added that "I believe this kind of brutality is intolerable to almost anyone."

Another victim of Saturday night's carnage, who had been listed "unknown remains," was identified today as Chabilall Jagarhauth, 22, a light-skinned native of Guyana. He, too, had been run over by a car, and his face had been badly mutilated. Medical authorities said he had been stabbed in the head and shoulder, and his right arm and ankle had been fractured.

Dade County's 253 schools, which have been closed all week, were to reopen Thursday. Rescinding the curfew also meant resumption of liquor sales. Remaining in effect are prohibitions on the sale of firearms, ammunition and gasoline and other flammables in containers. Gas can be sold only if pumped into vehicle tanks.

Police in Miami released records today showing that 372 persons were arrested during the two days of rioting and looting here. Most of the arrests, 233, were for looting, and most of the persons were black males; with some black women and white men.

Spokesmen for the Miami police department said that most of the suspects claimed they were unemployed. Those charged who did list employment usually held marginal jobs such as general labor, maintenance, and cooks.

The crimes allegedly committed by the suspects included disorderly conduct while intoxicated, 71; curfew violations, 46; carrying concealed firearms, 12; loitering-prowling, 12; arson by fire bombing, two; assault and aggravated battery, eight; grand larceny, two; auto theft, six; and strongarmed robbery, one.

Overall, there were 1,300 arrests in the Dade County area, which encompasses Miami, over the two days of rioting.

In Washington, Charles Morgan Jr., a lawyer who filed many civil rights cases in the South in the 1960s, said today that he was troubled by the Justice Department's actions, despite the gruesome facts of the McDuffie case.

"There is a very very serious policy question regarding federal prosecution of people who have been acquitted by a state jury, especially when the federal prosecution is announced in response to a riot," he said.

Justice Department officials say that the "dual prosecution" policy doesn't violate the "double jeopardy" clause of the Fifth Amendment which states no person "shall be subject for the same offense to be twice put in jeopardy of life of limb."

The Supreme Court has upheld the practice of allowing such double prosecutions, though the opinions included so many warnings against indiscriminate use that for years the Justice Department rarely sought indictments in cases where the state already had acted.

In early 1977, though, then Attorney General Griffin B. Bell issued a statement that was generally seen as expanding the Justice Department's ability to seek dual prosecutions in civil rights cases.

The case that triggered the pronouncement involved the killing of Richard Morales, in Castroville, Tex., in 1975 by the local police chief, Frank Hayes.

Hayes was prosecuted for murder under state law. The jury lowered the charge to aggravated assault, convicted him and sentenced him to 10 years in jail. The Hispanic community in Texas thought the verdict too lenient and demanded federal action.

Bell said he was approving the federal prosecution of the case because of "the exceptional circumstances," although the U.S. attorney in Texas recommended against it. Hayes was convicted and sentence to life imprisonment.