A method for a positioning server configured to locate positioning devices and to maintain information on users, who are associated with respective positioning devices. The server receives one or more alert messages from an alerting positioning device. Each alert message conveys identifying information of the alerting positioning device. The server uses the identifying information to retrieve a location estimate of the alerting positioning device. The server forms a first set of other positioning devices in a vicinity of the alerting positioning device and sends an assist request message to the positioning devices in the first set. The assist request message indicates the location estimate of the alerting positioning device and/or said descriptive information.