When a jury declared Allison Jenkins guilty of murder earlier this month, his face twisted into a grimace. ''What?'' he exclaimed.

A short time later, Andrea Grannum walked out of the Criminal Courts Building ahead of her 11 fellow jurors, tears streaming down her face. She feared that she had helped to convict a man of murder whose real crime was something far less.

Jenkins, 29, was found guilty of the murder of a police officer, though he had never pulled the trigger. The officer, Jay Brunkella, had been shot by his partner.

But prosecutors said Jenkins had struggled during an arrest for possession of marijuana, causing the partner`s gun to fire and kill Brunkella. Jenkins was therefore guilty under Illinois` ''felony murder'' law, they said. In essence, Jenkins` physical struggle with police during his arrest changed his initial crime from a misdemeanor to a felony. And the law holds felons responsible for deaths that occur during forcible felonies.

The case was highly unusual. And from the start, it was baffling to Jenkins.

''I`m basically average, you know, there are certain things I believe in,'' Jenkins said in an interview in Cook County Jail after the verdict.

''One thing that really, really puzzles me-how can they charge me with a murder I didn`t commit?

''How can a man not have a gun and be found guilty of murder? The question is there, How can it be done?''

It is a question that legal scholars have pondered for decades-a question that even some of Brunkella`s fellow officers have asked themselves.

Allison Jenkins, a native of Belize, was not an unfamiliar character to police in the Rogers Park district. Characterized by officers as ''a nickel-and-dime marijuana man'' who occasionally sold small quantities of drugs, at the time of the shooting he was out on bond, awaiting trial on a charge of selling 3 grams of cocaine to an undercover police officer.

Jenkins recognized officer Jay Brunkella as Brunkella and his partner, Fred Hattenberger, approached him outside a Rogers Park elementary school on Sept. 22, 1986, to arrest him for possession of marijuana, court testimony shows.

During Jenkins` trial, Hattenberger testified that he pulled his .45 Colt revolver and that Jenkins ran. Hattenberger said that when he caught Jenkins, the suspect still resisted arrest, struggling and then striking him in the chest with his elbow-a blow that he said left a bruise. Hattenberger said he lost his balance and the two men fell. His finger accidentally pulled the trigger; the bullet struck Brunkella, 39, who died two weeks later without regaining consciousness.

The felony-murder law that Jenkins is charged with violating is based in English common law, said Norval Morris, a University of Chicago professor of law and criminology. It stems from the premise that ''if you embark on a forcible felony (in Jenkins` case, aggravated battery on a police officer), you`re risking life'' and are responsible if someone dies, he said.

Typically, the law applies to armed-robbery murders or rape-murders. In more unusual cases, though, it can ''lead to results sort of out of line with our intuitive sense of justice,'' said John Donohue, associate professor of law at Northwestern University.

Morris said Jenkins could argue that his actions weren`t ''as evil as having an intent to kill, and a rational system would distinguish between them.''

Britain itself abolished the rule in 1957, Donohue said. He said several other states have narrowed it so a felon is responsible only for a killing committed by his own hand or by his ''agent,'' usually a codefendant.

''The most likely result around the country is that this (Jenkins`) case would not be felony murder,'' he said.

The original felony that Jenkins was charged with is unusual in itself. His possession of the bag of marijuana would have been a misdemeanor. But Hattenberger said Jenkins fought him during the arrest. Had he fought a civilian in the same way, the charge would have been misdemeanor battery. But battery against a policeman becomes aggravated battery, a forcible felony.

Battery thus became aggravated battery; aggravated battery became felony murder. ''This type of application is really pushing the felony-murder doctrine to the limits,'' Donohue said.

The state`s attorney`s office, though, is ''convinced that the charges we approved are appropriate,'' spokeswoman Lisa Howard said.

''The law holds a felon responsible for his actions when he sets into motion a chain of events that causes the death of another person,'' Howard said. ''The purpose of this law is to protect the community, the innocent, and to hold the felon accountable for any harm caused by his felonious activity.'' From Donohue`s post at Northwestern, the case is ''a fascinating felony-murder situation.'' To Jenkins, it`s a puzzle that has turned his life around. And to juror Andrea Grannum, a South Shore High School teacher, the case was an experience she wishes she never had.

Jenkins, who is black, said he believes he might not have been convicted had there been more than one black, Grannum, on the jury. Though Jenkins`

lawyer, Craig Katz, objected to the racial composition of the jury, Cook County Criminal Court Judge Joseph Urso ruled that there had been no deliberate racial exclusion.

As it turned out, Grannum was in jury deliberations for nine hours, arguing for Jenkins` acquittal. Finally, she gave in, she says.

''It`s a shame to say I needed support,'' she said later. ''It`s a shame to say I couldn`t do it myself.''

Surrounded by her 11 fellow jurors, she said she finally surrendered to their mounting pressure to convict Jenkins. Even as she did, ''I tried to see it (as murder),'' she said, ''but I just didn`t feel it.''

Jenkins had testified that he never elbowed Hattenberger in the first place-he said he was backing away from Hattenberger, saying, ''Don`t shoot,'' when Hattenberger used the weapon to strike him and the gun went off. Jenkins thus contended that he hadn`t committed the felony required for the felony-murder charge.

Grannum said it wasn`t hard for her to believe that a policeman could do what Jenkins said he did-that officers sometimes used unnecessary force. But her fellow jurors, most of whom live in suburban areas, were skeptical.

''They were completely out of touch with reality-with the way things can be

(in the city) and are,'' she said.

The other 11 jurors refused comment as they left the court, and the foreman later refused to discuss the case when reached at his home.

The night of the verdict, Grannum had a hard time concentrating on driving home. When she got there, she couldn`t sleep. The next morning, she talked to her clergyman. The next week, she called Judge Urso.

The verdict was wrong, she told the judge. As it turned out, she wasn`t the only one who thought so.