LIMA, Peru (AP) — Peruvian authorities say they are struggling to keep outsiders away from a clan of previously isolated Amazon Indians who began appearing on the banks of a jungle river popular with environmental tourists last year.

The behavior of the small group of Mashco-Piro Indians has puzzled scientists, who say it may be related to the encroachment of loggers and by low-flying aircraft from nearby natural gas and oil exploration in the southeastern region of the country.

Clan members have been blamed for two bow-and-arrow attacks on people near the riverbank in Madre de Dios state where officials say the Indians were first seen last May.

One badly wounded a forest ranger in October. The following month, another fatally pierced the heart of a local Matsiguenka Indian, Nicolas "Shaco" Flores, who had long maintained a relationship with the Mashco-Piro.

The advocacy group Survival International released photos Tuesday showing clan members on the riverbank, describing the pictures as the "most detailed sightings of uncontacted Indians ever recorded on camera."

The British-based group provided the photos exactly a year after releasing aerial photos from Brazil of another tribe classified as uncontacted, one of about 100 such groups it says exist around the world.

One of the Mashco-Piro photos was taken by a bird watcher in August, Survival International said. The other two were shot by Spanish archaeologist Diego Cortijo on Nov. 16, six days before Flores was killed.

Cortijo, a member of the Spanish Geographical Society, was visiting Flores while on an expedition in search of petroglyphs and said clan members appeared across the river from Flores' house, calling for him by name.

Flores could communicate with the Mashco-Piro because he spoke two related dialects, said Cortijo, who added that Flores had previously provided clan members with machetes and cooking pots.

The Mashco-Piro tribe is believed to number in the hundreds and lives in the Manu National Park that borders Diamante, a community of more than 200 people where Flores lived.

Although it's not known what provoked the Mashco-Piro clan to leave the relative safety of their tribe's jungle home, Beatriz Huerta, an anthropologist who works with Peru's agency for indigenous affairs, speculated their habitat is becoming increasingly less isolated.