17 days on, does Gaza clan hold the key?
There has been little outward signs of progress since the Scot was kidnapped by masked Palestinian gunmen on 12 March.
Many suspect that members of a powerful Gaza clan took him, but security officials - those loyal to both the Islamist group Hamas and its rival Fatah - are reluctant to pursue the matter, hoping not to antagonise a heavily armed family that could shift its loyalty to either side if infighting between the two groups begins again.
It's unsettling for foreign journalists already reluctant to cover events in Gaza, a coastal strip with a population of 1.4 million which is marked by violence and chaos.
Yesterday, the Tel Aviv-based Foreign Press Association (FPA) issued a warning to its members, saying the BBC had "convincing evidence" that Palestinian militants were planning to abduct more foreigners. It said: "We urge all members thinking of going to Gaza to re-evaluate the necessity of their trip and take all possible care."
Two employees of the American Fox News network were kidnapped and held for two weeks last summer, and several more have been snatched since then, including an Associated Press photographer. All were released unharmed.
The kidnappers typically demand money or jobs from the Palestinian Authority and go unpunished for their crimes - perhaps encouraging them to see abducting foreigners as a lucrative enterprise with few drawbacks.
Johnston, 44, who was educated at Dollar Academy and Dundee University, was the only western reporter permanently based in Gaza, where he has lived for three years. He had been preparing to leave at the beginning of April.
Last week, his father, Graham Johnston, called for his immediate release, saying: "It is no way to treat a friend of the Palestinian people. All I can say to the men that are holding Alan is, 'Please, let my son go. Now. Today'."
Suspicion has fallen on Gaza City's Doghmush clan, which has shifted its allegiance, and the backing of its private militia, between Fatah and Hamas - armed men loyal to both groups have waged deadly street battles since Hamas won elections last year.
Yehia Issa, a Palestinian official, insisted they were not stalling in the search for Johnston. "Security forces have to be cautious in order not to harm the kidnapped person," he said.
There have been only tentative signs of mediation on the kidnapping, and militant factions and government officials have said little on the matter.
Shuhdi Kashef, of the Gaza journalists' union, said his group had tried to fill the vacuum, sending delegations to meet security officials and powerful clans, including the Doghmushes. Journalists have held two strikes and several protests over the abduction of Johnston.
BBC staff say they've been told little about the situation. "We don't know who has got him, we don't know if there are demands, we don't know if there's any negotiations," said Joaquin Floto, a senior Middle East producer.
Many foreign reporters are now hesitant to cover stories in Gaza, Simon McGregor-Wood, the chairman of the FPA, said. "I think there's a great concern that the situation is getting worse, and Alan is a signal of that."