Mexico City, October 16 (NNN-RHC) -- Mexican authorities in the town of Iguala scrambled to find 43 students missing since gang-affiliated police attacked them 19 days ago, as angry protesters demanding their safe return threw rocks at the attorney general`s office.

The announcement that the young men were not among 28 bodies found in a mass grave in the southern city of Iguala, where they disappeared on September 26th, brought only temporary relief to families and fellow students.

Investigators have been digging at two other locations outside Iguala with burial pits holding an undisclosed number of bodies, and officials warn that identifying the victims will take time.

Hundreds of students protested in front of the attorney general`s office in Mexico City, hurling stones at its windows and igniting a bonfire in front of the gate.

"They took them alive, we want them back alive," the protesters chanted.

President Enrique Pena Nieto has been under pressure at home and abroad to solve the baffling case in the violence-plagued southern state of Guerrero.

Authorities say Iguala's police force shot at buses carrying the students on September 26th and handed them over to officers in the neighboring town of Cocula, who then delivered them to the Guerreros Unidos drug gang.

Six people were killed, 25 were wounded and 43 students went missing that night. Horse-mounted police and rescue dogs were deployed to comb rural, hard-to-reach areas of Iguala, 200 kilometers south of Mexico City.

Some 300 federal police officers and civilian self-defense militias have already been searching for the students for days. Attorney General Jesus Murillo Karam said Tuesday that 14 Cocula police officers were arrested in the case, joining 26 detained colleagues from Iguala.