Gary Cronin, a Ministry of Defence firefighter, was driving across the Erskine Bridge when he saw the pair, believed to have had an apparent suicide pact, sitting on the outer railing with their backs to the water.

He told a fatal accident inquiry at Paisley Sheriff Court they fell in a "very controlled" manner.

Mr Cronin stopped his car and ran to look over the edge of the bridge but could see nothing as it was "pitch black".

Niamh Lafferty, 15, and Georgia Rowe, 14, died after they fell more than 100ft on the evening of October 4 2009.

Mr Cronin was driving from the Renfrewshire side of the river to the Dunbartonshire side when he saw the girls.

He told the inquiry: "I was travelling across the bridge and I said to my girlfriend 'Look at the beautiful view' and she looked to the right.

"I looked forward and I saw two figures arm in arm and instantly they fell backwards in a very controlled manner."

Asked whether there was anything to suggest it was not a deliberate act, he said: "Not at all." When he stopped the car, his partner, a nurse, rang emergency services.

The girls were residents at nearby secure unit the Good Shepherd Centre in Bishopton, Renfrewshire.

Niamh, from Helensburgh, was placed at the unit in June 2008 after being taken into care.

Adah Lambie, 55, who at the time was a social work team leader at Argyll and Bute Council, said: "She was a troubled young person."

Ms Lambie said that during autumn 2008 the department had difficulties with Niamh's father, Paul Lafferty, whom she said could be unreliable.

However, she said the ultimate goal of the department was to see Niamh move back in with her father because that was what the girl had wanted.

The inquiry continues.