Persistence

An obvious trend over a 26-year period of the terrorist campaign is the decreasing percentage of Northern Ireland casework that is positive for FDR. During this period substantial improvements have been made in the efficiency of sampling and in the sensitivity of the detection techniques. Despite this, the downward trend continued. Our success rate decreased from approximately 35% at the start of the terrorist campaign in 1969 to about 6% (excluding suicides and dead suspects) in 1995. The reason for the decreasing success rate is not the detection system but rather the careful planning of terrorist incidents and the precautions terrorists take to prevent leaving any type of forensic evidence at a scene or on their persons. Coupled with this is the unfavorable behavior of FDR particles once they are deposited on a suspect. The particles are small and lightly adhering and, as such, can become table 20.13 Elemental Levels (ng) on Perimeter of Bullet Hole

Sample No./Jkt

Material

Pb

Sb

Ba

Cu

Ni

Hg

Comments

1 Ni

2,600

None

160

600

None

None

No Ni, low Pb and Ba

2 Cu

>10,000

None

500

1,150

None

None

No Sb. Ba present

3 Cu

>10,000

38

1,690

1,500

3,700

None

High Ni

4 Cu

>10,000

None

None

None

None

None

Pb only detected

5 Cu

8,300

None

850

>5,000

None

None

No Sb

6 Cu

>10,000

None

560

4,075

2,850

None

No Sb, high Ni

7 Cu

9,350

30

410

4,650

None

None

Pb and Ba present

8 Cu

9,000

None

650

>5,000

None

63

No Sb. Ba present

9 Cu

3,950

None

520

>5,000

None

None

No Sb

10 Cu

8,875

30

520

4,750

None

None

11 Cu

4,400

20

1,460

2,200

None

None

12 Cu

>10,000

46

670

2,600

None

None

13 Cu

>10,000

36

700

2,550

None

None

14 Cu

>10,000

33

540

4,175

2,000

160

Ba present, high Ni

15 Cu

>10,000

None

830

>5,000

1,900

None

High Ni

16 Cu

9,500

41

580

4,100

None

176

Ba present

17 Cu

3,850

66

130

3,100

None

286

Ba present

18 Cu

3,830

75

400

>5,000

None

None

19 Cu

>10,000

167

2,000

>5,000

1,725

None

High Ni

20 Cu

>10,000

137

1,470

3,275

None

None

21 Cu

5,150

102

650

4,890

2,300

>500

Pb and Ba present, high Ni

22 Cu

9,220

None

>2,000

>5,000

None

>500

No Sb

23 Cu

>10,000

52

None

>5,000

None

>500

24 Cu

>10,000

None

390

>5,000

None

None

No Sb

25 Cu

2,150

None

160

>5,000

None

None

No Sb, low Pb and Ba

26 Cu

5,450

None

450

2,490

None

None

No Sb

27 Cu

5,000

None

740

1,850

2,100

None

No Sb, Ba present, high Ni

28 Cu

3,375

29

450

1,400

None

None

29 Cu

4,900

None

>2,000

4,750

2,250

None

No Sb, high Ni

30 Steel

2,950

None

1,270

>5,000

2,150

None

No Sb, high Ni

31 Steel

>10,000

55

1,050

>5,000

2,150

None

High Ni

32 Cu

8,800

30

>2,000

>5,000

None

None

33 Cu

5,900

32

720

>5,000

None

None

Ba present

table 20.13 Elemental Levels (ng) on Perimeter of Bullet Hole (Continued)

Sample No./Jkt Material

Pb

Sb

Ba

Cu

Ni

Hg

Comments

34 Cu

4,700

>200

900

>5,000

None

155

Pb and Ba present

35 Cu

850

>200

550

>5,000

None

>500

Pb and Ba present

36 Pb

>10,000

>200

>2,000

3,950

None

>500

37 Pb

>10,000

>200

>2,000

3,325

None

>500

38 Cu

>10,000

>200

>2,000

>5,000

2,850

None

High Ni

39 Cu

>10,000

None

240

2,200

None

None

No Sb

40 Cu

7,300

None

200

1,100

1,600

None

No Sb, Ba present, high Ni

41 Pb

>10,000

>200

520

None

None

None

42 Pb

>10,000

None

835

None

3,675

None

No Sb, high Ni

43 Ni

>10,000

None

390

>5,000

3,250

>500

No Sb, Ba present, high Ni, Cu

44 Pb

>10,000

>200

1,830

>5,000

2,850

>500

High Ni

45 Pb

>10,000

174

1,770

3,300

4,750

>500

High Ni

46 Pb

>10,000

174

700

900

None

None

47 Pb

>10,000

130

1,370

1,675

None

None

airborne again and be transferred from surface to surface by physical contact. They are lost rapidly from the hands, an order of magnitude in the first hour, and consequently the detection of residue on the hands suggests very recent contact (excluding suicides and dead suspects). They persist longer on clothing surfaces, the length of time depending on the nature of the material and the extent of physical disturbance of the garment. The particles are chemically stable. This was confirmed by an experiment involving an FDR-contaminated garment which was packaged, sealed, and stored for 2 years. FDR particles were readily detected on the garment surface after the lengthy storage period.

Another persistence experiment involved prompt sampling of the firing hand after firing. Numerous FDR particles were detected on the firing hand. The experiment was repeated but the firer was allowed to dry wipe his hands on tissue, in an effort to remove any FDR particles, prior to sampling. Very few particles were detected and it was concluded that FDR particles can easily be removed from the hands, even by dry rubbing.

Statistics gathered from 15 years of casework results gave the following persistence data. Figure 20.1 illustrates the situation for suspects whose hands, face, and head hair was sampled for FDR, resulting in the detection of particles on all or some of the samples. Suspects are rarely apprehended

Time between Incident and Apprehension (hours) Probability

Clothing Hair Face Hands Significance

Time between Incident and Apprehension (hours) Probability

Clothing Hair Face Hands Significance

Figure 20.1 Persistence of FDR.

immediately. The data are based on 410 positive swab kits and exclude suicides and dead suspects.

It is difficult to produce valid persistence data for clothing other than to say that it is our most fruitful sampling area (pocket interiors in particular). As stated previously, persistence on clothing depends on the nature of the material and the degree of physical disturbance the garment suffers. FDR will remain indefinitely on clothing if the clothing is undisturbed.

Suspects have been known either to abandon or to destroy clothing worn at the time of the incident and change into "clean" clothing. Consequently it is often not known for certain if the clothing submitted to the laboratory was the clothing worn during the incident.

FDR has been detected on the clothing of a suspect up to 6 days after an incident, but the history of the clothing was not known; consequently the residue could have been deposited since the original incident. On the other hand residue found on a garment could have originated from an incident prior to the incident under investigation. This highlights the problems encountered when interpreting positive results on clothing. Residue detected on the hands, face, or head hair, because of known persistence, can be assumed to be recently deposited whereas on clothing (particularly pocket interiors) it is difficult to link it to a specific shooting incident. However, it is still valuable evidence requiring an explanation from the suspect.

FDR is not likely to be found on the hands if the time between the incident and apprehension exceeds 2 hours (the suspect's hands must be protected immediately after the suspect is apprehended). The police are instructed not to sample the hands if the 2 hours are exceeded but to take the face and head hair samples as normal. FDR has been detected on the face up to 5 hours and on the head hair up to 7 hours after an incident.

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