BANGKOK (Reuters) - Thailand on Friday indicted 72 people suspected of involvement in human trafficking, just days before the United States issues its latest assessment of the country's anti-trafficking efforts next week.

Bangkok launched a sweeping investigation into human trafficking in May after the discovery of 26 bodies in graves buried deep in a jungle near the Thailand-Malaysia border.

The clampdown triggered a regional migrant crisis because it prompted criminals to abandon boats crammed with thousands of migrants at sea, rather than risk landing on Thai shores.

The investigation was the "biggest into human trafficking" in Thailand's history, deputy national police chief Aek Angsananont said.

Police said they have so far arrested 72 people and issued arrest warrants for 45 more.

"We will indict all 72 people who have been arrested," Wanchai Roujanavong, a spokesman for the attorney general's office, told a news conference.

"For those people who are still on the run overseas, we will work with national police to send them back, according to existing extradition treaties."

Fifteen of those arrested were Thai officials, he said, including one military soldier and four police officers.

They have been charged with crimes ranging from human trafficking and transnational criminal activity to bringing foreign workers into Thailand illegally and violation of official duty.

Case files have been sent to a court in the southern province of Songkhla, where the migrants' graves were discovered, and where the suspects are jailed, Wanchai added.

The court must now decide whether to take up the cases.

Despite the arrests, questions have been raised about the long-term effectiveness of Thailand's crackdown on the lucrative trafficking syndicates.

A Reuters investigation this month showed how the crackdown ran into daunting obstacles, including witness intimidation.

Last June, the United States downgraded Thailand, one of its oldest treaty allies in Asia, to the lowest "Tier 3" status in its 2014 Trafficking in Persons Report, for not meeting the minimum standards for combating the illicit trade.

Until then, Thailand had been on the report's so-called Tier 2 Watch List, the second-lowest rank, for four years straight.

A Tier 3 rating would normally trigger a range of sanctions by the United States but President Barack Obama waived them in Thailand's case.

Washington releases a new report on Monday.