Officers recovered several hundred of the fireworks from a shed following the incident, but believe others may have been taken by children in the run up to Bonfire Night on November 5.
The search is focused on the North Lanarkshire area after the 15-year-old was rushed to hospital after one of the devices exploded in his hand at about 5.20pm yesterday.
He is being treated in a specialist burns unit at Glasgow Royal Infirmary following the incident in Motherwell. His condition was last night described as stable.
About two hours earlier, a 12-year-old boy suffered superficial burns to his hand after being scorched by a banger in a street in nearby Carfin.
He was taken to Wishaw General Hospital, where he received treatment, but was later released.
Inspector Gavin Russell, of Wishaw Police Office, said he feared hundreds of the faulty fireworks could be in circulation locally.
He also appealed for youngsters to take great care when handling fireworks.
He said: "From what we can establish at the moment the banger fireworks were stored in a shed behind houses in Blackmoore Place, New Stevenson. So far, we have recovered several hundred but believe there could have been up to 1000 hidden there.
"At this time we do not know who the fireworks belong to or indeed who owns the shed they were in. It is very possible that they have been stolen from another premises and stored there, with or without the owner's knowledge. However, this is still to be established. What concerns us at the moment is that they now appear to have got into the hands of local youngsters in the Newarthill, Holytown, Carfin and New Stevenson areas.
"It is too early to say whether the injuries the boys sustained are due to the way they have been handled or if the batch is defective. However, my main point here is to make sure that anyone else who comes across them is not injured.
"I would urge people who are either offered them to play with or to buy, to not use them but to hand them in to police.
"I would also appeal to anyone who has information about the fireworks, where they came from or who they have been circulated to, to contact officers."
Information can be passed to Bellshill Police Office on 01698 202 400.
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