An aphorism I jotted down in my handy breast-pocket notebook holds that “in photojournalism the picture illustrates a story, and in art photography a picture is the story.” This may even be true. But there is another category of pictures that start out as photojournalism and become something else when the story they set out to illustrate is forgotten and the images float like stills from a lost movie, clear moments in a tale of obscure beginnings and unsure consequences, but no less dramatic for being suspended in a narrative limbo. Many of the 92 photographs in “Mexico: The Revolution and Beyond, Photographs by Casasola 1900-40,” a remarkable exhibition at El Museo del Barrio, are in that category.

For instance in “The Body of Hazel Walter, a North-American Who Committed Suicide” (c. 1925), the exquisite body of a beautiful young woman lies naked on a concrete block, her hands resting folded on her stomach. The camera is low enough for us to see her classic Anglo-Saxon features in profile, her blond hair, and her dark pubic hair, the rough concrete floor, and two caskets out of focus in the distant background. Where are we? Probably in a morgue, a mortuary, some place devoted to death. But who was Hazel Walter? What was she doing in Mexico? And why did this lovely, elegant woman kill herself? A whole slew of noirish scenarios spin themselves out but, as Bob Dylan sang nastily, “Something is happening, but you don’t know what it is, do you, Mister Jones?” We are excluded by time, by distance, and by ignorance.

Agustin Victor Casasola (1874-1938) was one of Mexico’s earliest photojournalists. Beginning as a reporter, he established himself as a photographer by the turn of the century, and in 1912 opened one of the country’s first professional photography agencies, Agencia Mexicana de Informacio Fotografica. His motto was, “I have or will make the photo you need.” Eventually his brother, his children and grandchildren, and maybe as many as 400 other photographers worked for the agency. Casasola had a passion to record the realities of his time and country, shot everything, went everywhere, and kept whatever he had.

The Casasola Archive, from which the pictures in this exhibition were selected, is stored at the National Institute of Anthropology and History; its holdings include 483,993 items. But individual ascriptions are lost, or rather Casasola ascribed all the pictures to himself, so they are all effectively anonymous. That adds to the aura of ambiguity in which these images are suspended. All the prints are new, but many of the negatives are damaged and the cracks and deterioration show up to give a further patina, making them appear as much archaeological artifacts as works of art: that they certainly are. Mexico in the first half of the 20th century becomes vividly real – violent, tender, comic, tragic – its people not the sleepy caballeros we are accustomed to from comic strips and movies, but totally plausible.

The violence comes from political turmoil. The first sections of the exhibition are devoted to the dictatorship of General Porfirio Diaz and the 10 years of war that followed the outbreak of revolution in 1911.”President Porfirio Diaz at an Homage to Benito Juarez, ‘Glorious Son of the Motherland,’ Mexico City” (July 1910), shows him sitting on a dais in a gilded chair with a snake and serpent in bas relief on its backrest, surrounded by dignitaries in formal wear. A silver dinner bell is the sole object on the table before him, there as if for him to summon staff to remove whatever displeases him. His face is handsome, intelligent, and brutal.

There are several pictures of troops. “Soldiers on a Locomotive” (c. 1915) shows these men posing on the cowcatcher of a train that will take them to an uncertain fate? Some wear uniforms, most are shabby, a civilian to the right is in a suit and straw boater; on the left are two young boys, one unarmed, one with a rifle, bandolier, and a peaked hat to show he is a soldier. “Soldadera” (c. 1915) is a sitting portrait of a woman soldier, the handle and cartridge chamber of a revolver nonchalantly sticking out of the pocket of her striped pants. Her jacket is decorated with medals and ribbons, and a straw hat frames her dark, stoic face. I do not know on which side or in which war she fought.

“Colonel Leon Roman During Target Shooting Practice, Mexico City” (c. 1924) has him standing erect in a smart uniform draped with whipcord, belts, and a holster, as he points his pistol past us. That picture hangs effectively next to “Fortino Samano Smoking Before Being Executed By Firing Squad” (January 12, 1917): A tall, thin man, he stands casually with his hands in his pockets and a cigar between his clenched teeth. The same rough wall in that picture is in “Execution of the Banda del Automovil Gris or ‘Gray Automobile Gang,’ Mexico City” (1919): four figures with their knees buckling, blurred as they fall so none of their features are visible, although none are needed. All I know about any of these people is what can be read in the Casasola images.

The Work Place section has the dignified men of “The National School for the Blind Orchestra, Mexico City” (c. 1910) posed with their instruments,and the proud “Executives and Employees at the Ford Car-Assembly Plant, Mexico City” (c. 1927) stand behind an uncompleted chassis. The Modern Times section has “Typists Blindfolded for an Exam, Mexico City” (c. 1920), rows of women at newfangled typewriters all with their faces covered: surreal. The Night Life section has the campy men of “Arrested Homosexuals Pose for the Camera at the Police Station, Mexico City” (c. 1935), and the six chubby lovelies of “Ba-Ta-Clan Chorus Girls, Mexico City” (c. 1925). The girls wear tights and have big cardboard stars on their heads with their faces sticking out of the middle.

There is much more in this exhibition, including three pictures of the romantic revolutionary Emiliano Zapata, (two alive, one dead) and one of Agustin Victor Casasola himself. Casasola’s obsession left his country an important visual heritage. The compelling quality of these pictures leaves us grasping for strands of stories that took place just south of our border.

Until July 31 (1230 Fifth Avenue, at 104th Street, 212-831-7272).