U.S. Faces Obstacles in Bid to Arrest 2 in Big Trading Loss at JPMorgan Federal investigators, seeking to bring criminal charges against two former JPMorgan Chase employees at the center of the bank's multibillion-dollar trading loss in London last year, are facing logistical hurdles in planning arrests. One of the employees remains on vacation while the other has returned to his native France, complicating plans to extradite them under an agreement with British authorities. A third employee, Bruno Iksil, has reached a so-called nonprosecution deal with federal investigators in Manhattan that will shield him from charges as long as he cooperates against his two former colleagues, people briefed on the matter said. One of those colleagues is Javier Martin-Artajo, who oversaw the trading strategy from the bank's London offices. Lawyers for Mr. Martin-Artajo broke their silence on Tuesday to say that he was "currently on a long-planned vacation and will be returning" to London "as scheduled." The lawyers, from Norton Rose Fulbright, declined to say where he was or when he would return, adding that the authorities had not instructed him to postpone his vacation. The other colleague, Julien Grout, left London this year for France, which typically does not extradite its citizens. Mr. Grout's lawyer, Edward Little, explained that his client moved after losing his job with JPMorgan last December. Mr. Grout, his lawyer said, transferred his belongings to France and spent a brief period in the United States, where his wife's family lives. The move occurred long before news media reports surfaced last week that Mr. Grout could be arrested, Mr. Little said. "He has absolutely no intention of fleeing," Mr. Little said. The authorities need not wait for the employees to return to London to bring charges. Even without arrests, people briefed on the matter said, prosecutors and the Federal Bureau of Investigation in Manhattan could announce the charges this week. The authorities spent more than a year investigating Mr. Martin-Artajo and Mr. Grout in connection with their roles in the loss at JPMorgan. The people briefed on the matter said the authorities suspect that Mr. Martin-Artajo instructed lower-level traders, including Mr. Grout and Mr. Iksil, to mask the size of the mounting losses last year. Scouring internal e-mails and telephone records, the authorities said they came to believe that the traders had falsified internal bank records. JPMorgan eventually restated its first-quarter earnings for 2012, adjusting them down by $459 million to concede that the valuations were flawed. Yet, Mr. Martin-Artajo's lawyers said he "is confident that when a complete and fair reconstruction of these complex events is completed, he will be cleared of any wrongdoing." Some legal specialists have noted that the prosecutors face challenges in proving their case. Traders have some wiggle room to value their trades on derivatives contracts because the actual prices might not be immediately available. That leeway could pose a challenge for prosecutors who will have to prove that the traders intentionally masked the losses. The case stems from a bet the traders built over many months. Deploying derivatives - complex financial tools with values linked to an asset like a corporate bond - the traders made bets on the health of large corporations like American Airlines. Those bets, which roiled the market and earned Mr. Iksil the moniker the London Whale, began to sour last year. The losses, which JPMorgan initially disclosed last May, have since swelled to more than $6 billion. The investigation into the losses accelerated with help from Mr. Iksil. The trader, despite receiving public blame for the trade, gave interviews to the United States authorities, at a meeting in Brussels and later in New York. Before agreeing to visit New York, according to the people briefed on the matter, Mr. Iksil secured the nonprosecution agreement. Under the deal, Mr. Iksil will not face criminal charges or a civil penalty from regulators as long as he cooperates in the case against the other traders. Even as prosecutors prepare criminal charges against the other two traders, they are continuing a parallel investigation into JPMorgan's failure to thwart what is suspected to be misconduct. The bank could face a reprimand and a fine from the authorities. Beside the criminal case, the Securities and Exchange Commission is investigating the trading losses. In an unusually aggressive stance for the agency, the people briefed on the matter said, the S.E.C. is looking to extract an admission of wrongdoing from JPMorgan as a condition of any settlement. Such an admission would be a change for the agency, which for decades had allowed defendants to "neither admit nor deny wrongdoing." Together, the cases swing an unwelcome spotlight back on JPMorgan, just as the bank, the nation's largest, is grappling with an array of regulatory woes. At least eight federal agencies are investigating the bank. One area of particular focus is the bank's mortgage business from the financial crisis era. Last week, JPMorgan disclosed that federal prosecutors in California were investigating whether it sold shoddy mortgage securities to investors before the 2008 financial crisis.