What's the form and type of the three verbs highlighted in the sentence below?

A helicopter carrying elite paramilitary mountaineers took off Wednesday for a "very high-risk" operation to retrieve five dead climbers and three others believed killed scaling a treacherous Himalayan peak.

Solution 1:

= (who were) believed (to be) killed (while they were) scaling [that peak]

The first two items are past participles used for passive voice (with some words omitted), the third one can be interpreted as an element of Past Continuous tense with 'while they were' omitted or simply as a gerund in the adverbial clause 'at/while scaling' with 'at/while' omitted.