The physical traces were observed, the witness was interviewed, photographs were taken, and soil samples were gathered according to standard police procedures. A few days later these samples were indeed forwarded to GEPAN while vegetal samples were sent to a laboratory of National Institute for Agronomic Research (INRA).

A short time later complementary samples were collected at the request of the analysis laboratories, as follows: (a) reference vegetal samples, gathered by the Gendarmerie on January 23rd and (b) a complete series of vegetal samples gathered by GEPAN itself on February 17th.

The decision to intervene was made by GEPAN because this particular case presented two types of information that could be confronted and correlated, namely the single witness report and the physical traces observed at the site. GEPAN does not routinely investigate cases when a single source of information is present because in such instances the investigator is unable to make comparisons and inferences among data coming through different channels.

Under the methodology used by GEPAN, to each type of available information there correspond a set of appropriate analysis techniques. Physical traces only constitute a useful channel of information to the extent that the analysis can be conducted before these physical effects have been dissipated. Among other requirements this involves securing the site rapidly and obtaining rigorously controlled samples under normal weather conditions. These minimal prerequisites were found to be satisfied in the Trans-en-Provence case: Accordingly GEPAN proceeded with its full investigation.

3. Environment of the Site

The village of Trans-en-Provence is located 3 km south of Draguignan. Mr. and Mrs. Nicolai live on a property situated 2 km east of Trans, on the side of a hill overlooking the valley of the Nartuby river (Figure 1). This valley is oriented east-west and contains numerous orchards and small agricultural plots. The sides of the valley are covered with woods and with Mediterranean vegetation. The sparse dwellings have their openings (windows, doors) facing the valley. Many terraces dug into the hillside make possible more intensive use of the soil for cultivation. Retaining walls, locally known as "restanques" and generally built out of native stone, have an average height of two m.

The plot of land owned by Mr. and Mrs. Nicolai is located some distance away from the D47 road, on the north side of the valley. A dirt road runs along the property. It ends 400 m away at an isolated farmhouse. The property is structured in such a way as to make the best possible use of the contour of the hillside. The house is built over several levels of retaining walls and it is partially dug into the hill. The driveway is covered with asphalt at the level of the basement, 30 m away from the road running to the west. On the left side