A Turkish court ruled to formally arrest five of eight people detained at the weekend for alleged fraud in transactions of banking shares on the Istanbul stock market, local media reported late on Tuesday.

Banking shares on the Borsa Istanbul Stock Exchange (BIST) have fallen sharply in recent weeks, after surging more than 150% between the start of the year and mid-September. The decline was largely due to margin calls on futures positions.

According to the prosecutor’s office investigation, a total of 10 people made high-volume transactions in banking stocks and futures contracts, giving the impression of high demand and artificially raising prices, Anadolu Agency (AA) reported.

Three suspects detained at the weekend were released, with one to be kept under house arrest and the two others forbidden from leaving the country, it said.

Aside from the eight people detained at the weekend, the two other suspects were abroad.

The investigation showed the suspects realized a TL 2.5 billion ($135 million) profit, before losing TL 1 billion when a sharp decline in prices began in mid-September.

The Capital Markets Board (SPK) said a criminal complaint was filed against 10 people over Borsa Istanbul Futures and Options Market (VIOP) transactions, with the same people and a construction company banned from trading for two years.

A sudden reversal in Türkiye’s stock market saw banking equities drop 39% in a three-week long rout that started on Sept. 13, according to Bloomberg News, threatening some brokerages’ financial stability. It estimated that the rout had erased about $14 billion in market value.

Analysts had said Ankara might move to limit futures contracts on shares of small or medium-sized companies with low trading volume vulnerable to price manipulation.

Investors in futures contracts have to meet margin calls by showing cash, shares or other securities equal to the margin call to keep their trading positions. Otherwise, brokerages meet them by selling the shares.