Tengku Sariffuddin, Mr. Najib’s press secretary, said in a statement issued early Thursday that Malaysia would cooperate with “any lawful investigation.” If any wrongdoing is proved, he said, “the law will be enforced without exception.” He said the investment fund had been the subject of several investigations in Malaysia and that authorities found no crime was committed.

The immediate effect of the Justice Department’s action is effectively to freeze the assets. The forfeiture process is lengthy. First, a court must make sure that no other interested party has a valid claim to the properties. Once true ownership is determined, the court must decide whether the money used to buy those assets was, in fact, earned illicitly. Only then can the government permanently seize the assets.

In the nearer term, the government’s action may increase the political pressure on Mr. Najib, who has held on to power by halting investigations into the investment fund and by removing officials in his governing party who criticized him.

The 1MDB fund was created in 2009 as a “strategic development company” to invest the Malaysian public’s money, primarily its oil wealth, in projects to benefit the country. Mr. Low helped set up an earlier version of the fund.

Image One of the targeted properties is the Montalban House in Los Angeles.

The Times reported last year that Mr. Low was secretly involved in major transactions with a small oil company called PetroSaudi International and Malaysia’s public fund, which was led by Mr. Najib in his position as prime minister. Mr. Low then helped the prime minister’s stepson buy property in the United States using shell companies and finance his movie company.

The Times investigation documented property transfers between Mr. Low and Mr. Aziz, including one on Hillcrest Road in Beverly Hills that was done behind the veil of a shell company, with no property transfer filed in public records, as well as the role that Mr. Low played in helping to finance Mr. Aziz’ movie business.