COCULA, Mexico — Mexican investigators planted red flags to mark suspected human remains Tuesday as they combed through a garbage-choked ravine in the search for 43 students missing since they were arrested by the police in the southern city of Iguala a month ago.

Yet as the second day of the search outside this forested mountain town closed, there was no sign that the government was any closer to finding the students.

On Monday, Attorney General Jesús Murillo Karam had raised hopes that the dump outside Cocula might hold a clue to the fate of the students, all young men, who the authorities said were handed over to a local drug gang after their arrests.

Four suspected members of the gang, known as Guerreros Unidos, had been arrested and provided information on Monday that led them to Cocula, about 30 minutes from Iguala.