Senators angered by the decision to upgrade Malaysia in the State Department’s human trafficking report have threatened a subpoena on Thursday unless they receive all communications related to the annual US ranking of countries’ efforts to combat modern-day slavery.

Last month’s report raised Malaysia from the lowest ranking, a change that critics say was related to the country’s involvement in negotiations for the Trans-Pacific Partnership, a 12-nation trade pact that Barack Obama is eager to conclude.

During a visit to Malaysia for regional security talks, secretary of state John Kerry told reporters that Malaysia had significantly improved combating what often is described as modern-day slavery. Kerry said he signed off on the report and had no conversations with anyone about links between it and the trade talks.

Later, at a Senate hearing, the official who oversees the department’s Office to Monitor and Combat Trafficking in Persons faced detailed questioning about the rankings for Cuba, India and Malaysia, in particular.

Members of the Senate foreign relations committee made clear their dissatisfaction with the answers from undersecretary of state Sarah Sewall. In Malaysia’s case, she cited legal reforms and increases in trafficking investigations and prosecutions during 2014.

“This is possibly the most heartless, lacking of substance presentation I have ever seen about a serious topic. I don’t see how anybody could believe that there was integrity in this process,” said the Republican chairman, Senator Bob Corker.

Millions of migrants from poorer Asian countries work in Malaysia in the flesh trade, on plantations and in domestic service. In May, Malaysian authorities found abandoned jungle camps used by human traffickers that held more than 100 graves suspected to contain the remains of Rohingya Muslims from Myanmar. Earlier, there was a similar discovery by Thailand police.

Thailand was among a group of 23 countries, including North Korea, Syria and Iran, that earned the lowest ranking in the most recent trafficking report.

Senator Robert Menendez, a Democrat, raised suspicions about political interference in Malaysia’s upgrade. Menendez, a trade pact opponent, had led the successful push for inclusion in recent trade legislation of a provision that bars the US from entering into trade agreements with nations with the lowest trafficking ranking.

Menendez called for the committee to review all the documentation created in relation to this year’s trafficking report. Corker said that if the State Department did not comply immediately, the committee would subpoena the information.

A State Department spokesman, Mark Toner, told reporters after the hearing that the department was waiting for the committee to submit a formal request “but speaking generally, of course we try to be responsive to Congress”.

Sewall said Malaysia had consulted with civil society groups and drafted amendments to its anti-trafficking law in order to better protect victims. She said trafficking investigations increased by more than 100% and prosecutions by 67%.

But Senator Ben Cardin, a Democrat, noted that the legal reforms had yet to be implemented and that trafficking convictions had dropped from nine in 2013 to just three in 2014. Sewall also acknowledged that the number of convictions was “inadequate”.

“Seems like you are trying to justify a result that’s not there,” he said.

The Obama administration also has drawn criticism for upgrading Cuba, a shift that came just a week after the US and Cuba formally restored diplomatic relations, ending a half-century of estrangement.

Speaking about Malaysia’s case, Kerry said that despite its upgrade, the government in Kuala Lumpur had “a long way to go” and that it did not mean a “gold seal of approval” from Washington. Malaysia will be demoted next year if it fails to follow through, he said.

“We all need to be true to the principle that although money may be used for many things, we must never allow a price tag to be placed on the heart and soul and the mind of a living person,” Kerry said.