Montelongo was last seen playing outside of his family's residence in San Benito, Texas on July 12, 1977. His mother, Enedelia Cavazos, called police that afternoon to say her son had disappeared. An extensive search failed to find him. A neighbor said he had seen Montelongo walking along Alamo Street, but this sighting has not been confirmed. Cavazos said he was on his way to see his aunt when he disappeared.

Montelongo enjoyed playing near an area drainage ditch. At the time of his disappearance the water level in the ditch was high, so investigators theorized he could have fallen in and been swept away. If so, his body was never found.

One of Montelongo's brothers died of Sudden Infant Death Syndrome (SIDS) about one year before Montelongo vanished. Another of his brothers may have died a few months after Montelongo was reported missing. Cavazos left the area in the middle of the investigation and lost touch with police. She did pass a polygraph in connection with Montelongo's case. She has had several children by different men and apparently has never married. Authorities do not know who Montelongo's father is. Cavazos now lives in Grand Rapids, Michigan under another name.

Due to the uncertain circumstances in Montelongo's disappearance, his case is classified as a non-family abduction. The case has been open but inactive since 1973. Authorities stated Cavazos was the primary suspect in her son's disappearance. Montelongo's case remains unsolved.