HALLETT, Okla. (AP) _ Federal experts arrived Wednesday to aid the investigation of a fireworks factory blast that killed 21 people, at least three of them 18 or younger, and Red Cross counselors were sent to help survivors.

There is ''nothing to indicate it was anything other than accidental,'' said Bob White, resident agent of the federal Bureau of Alcohol, Tobacco and Firearms.

But Paul Renfrow, spokesman for the Oklahoma State Bureau of Investigation, said the agency was investigating to ensure there was no criminal wrongdoing.

The victims, 14 females and seven males, died instantly in Tuesday's series of blasts and flash fire at the Aerlex Corp. plant 40 miles west of Tulsa, investigators said. Five men who were outside survived.

Plant owner Richard Alan Johnson, was among the survivors but would not talk with reporters. He was hospitalized with cuts.

An ATF national response team of experts arrived at the site Wednesday to help local agents, said White. ''They'll be going through and sifting through ... to find out what triggered this thing,'' he said.

The explosions which were heard at least 30 miles away and rattled windows 13 miles away took only 15 to 20 seconds to destroy three wood-frame and tin buildings in a ''domino effect all the way through the thing,'' White said.

The explosion and heat hindered identification of the bodies, said David Highbarger, investigator for the state medical examiner's office. Workers found shattered bodies as far as 200 yards away.

''We used dental records, old X-rays, personal identification papers, anything we could,'' he said.

Temperatures were so high that in some cases victims' teeth were incinerated, making identification from dental records nearly impossible, medical officials said.

People under the age of 18 are forbidden by law from working at such plants, but the medical examiner's list of identified victims included three aged 13, 17 and 18. State Labor Department officials went to the scene. The ages of two victims were not immediately known.

''It was a real popular spot for teen-agers to work and make some good money,'' said Mary Lewis, whose mobile home less than a mile away was shifted off its foundation by the blasts.

The plant had been busy preparing for the Fourth of July. It had a normal staff of about 10 but had expanded to nearly 30.

''They worked until late at night, and then started again real early in the morning to make all of the fireworks,'' said Ms. Lewis.

Cheryl Blonsky, spokeswoman for the American Red Cross in Tulsa, said counselors had been sent to help survivors deal with the tragedy. ''Not only the victims' families but also the workers who were out there yesterday will also have some kind of delayed reaction,'' she said.

''Everyone is still in a daze,'' said the Rev. George Boston, pastor of the First Baptist Church of nearby Cleveland.

White said initial investigations narrowed the location of the first explosion to a corner of the complex where a pickup truck apparently was being loaded.

State Fire Marshal Fred Rucker said he believes the explosion originated ''either in, on or adjacent to that pickup,'' but said the only witnesses had been killed.

''You have something of that magnitude, you don't have a lot of people to talk to,'' Rucker said.

Phillip J. Chojnacki of Houston, head of the ATF's national response team, said it was the nation's worst fireworks factory explosion in terms of deaths. The previous high, he said, was 11 killed in May 1983 at an illegal factory in Benton, Tenn.

It was the second explosion in six years at the plant, which opened in 1973, authorities said. A 1979 blast, which caused no injuries, was blamed on sunlight reflecting off a car mirror onto combustible material.

Gov. George Nigh ordered flags flown at half-staff. ''All Oklahoma is grieved for this tragedy,'' he said.

In addition to plant owner Johnson, two other men remained hospitalized Wednesday, one in critical condition. Two others were treated and released.