BVerfGE 109, 279 <351, 352>; 120, 274 <329, 334>). These measures constitute
such a serious interference with privacy that they cannot be extended to other persons. It is not constitutionally objectionable for measures targeting the persons responsible to also cover third parties, so long as this is inevitable (cf. BVerfGE 109,
279 <352 et seq.>). Thus, the surveillance of the home of a third party may be authorised, if on the basis of certain facts it can be supposed that the target person will be
present while the measure is carried out, will conduct conversations relevant to the
investigation there, and the surveillance of that person’s own home would not in itself
be sufficient for investigating the factual circumstances (cf. BVerfGE 109, 279 <353,
355 and 356>). Likewise, a remote search may be extended to the information technology systems of third parties if factual indications suggest that the target person
has saved information relevant to the investigation there and access solely to the target person’s own information technology system would not be sufficient for achieving
the aims of the investigation.
The ordering of other covert surveillance measures directly targeting third parties is
not impermissible per se. It is conceivable that the surveillance of persons – to be
clearly defined – in the target person’s sphere be authorised, for instance with regard
to contacts or messengers. The justification for such authorisation lies in the objective
nature of protecting against threats and of truth-finding in criminal investigations. The
extension of such an authorisation to third parties is subject to strict proportionality requirements and requires a specific individual proximity of the person concerned to the
threat or criminal offence being investigated. In that respect it is not sufficient that
there merely be some sort of contact with the target person. Rather, further indications are needed showing that the contact is relevant to the object of the investigation
and that there is thus a non-negligible probability that the surveillance measure will
contribute to elucidating the threat (cf. BVerfGE 107, 299 <322 and 323>; 113, 348
<380 ad 381>). The surveillance of persons that – based merely on the fact that they
have been in contact with the target person – attempts to find out whether this can result in further evidentiary bases for further investigations, is constitutionally impermissible. With regard to these contact persons, however, the Constitution does not bar
investigative measures that entail a lower level of interference from aiming to thereby
attain the threshold for surveillance measures entailing a higher level of interference.
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2. Overarching procedural requirements also derive from the principle of proportionality. The investigative and surveillance measures in question here, which predominantly involve serious interferences, and regarding which it can be presumed that
they will also record highly private information, and that are carried out covertly without the knowledge of the affected persons, as a rule require prior review by an independent body, in the form, for example, of a judicial order (on this, see also ECtHR,
Klass and others v. Germany, judgment of 6 September 1978, no. 5029/71, para. 56;
ECtHR (Grand Chamber), Zakharov v. Russia, judgment of 4 December 2015, no.
47143/06, paras. 258, 275; ECtHR, Szabó and Vissy v. Hungary, judgment of 12 January 2016, no. 37138/14, para. 77). For measures relating to the surveillance of pri-
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16/71