King Richard III's rediscovered resting place is turning out more mysteries this summer. Excavators finally lifted the heavy lid of a medieval stone coffin found at the site in Leicester, England, only to reveal another lead coffin inside.

The "coffin-within-a-coffin" is thought to have been sealed in the 13th or 14th century — more than 100 years before Richard, an infamous English king slain in battle, received his hasty burial in 1485.

The team of archaeologists from the University of Leicester thinks this grave in the Grey Friars monastery might contain one of the friary's founders or a medieval knight. [Gallery: In Search of the Grave of Richard III]

"The inner coffin is likely to contain a high-status burial — though we don't currently know who it contains," reads a statement from the university.

The outer stone coffin measures about 7 feet (2.1 meters) long and 2 feet (0.6 meters) wide at the head and 1 foot (0.3 meters) at the feet. Eight people were needed to remove its lid.

The lead funerary box inside has been carried off to the university, where researchers will conduct tests to determine the safest way to open it without damaging the remains. But so far, they've been able to get a look at the feet through a hole in the bottom of the inner coffin.

The archaeologists suspect the grave may belong to one of Grey Friar's founders: Peter Swynsfeld, who died in 1272, or William of Nottingham, who died in 1330. Records also suggest "a knight called Mutton, sometime mayor of Leicester," was buried at the site. This name may refer to the 14th-century knight Sir William de Moton of Peckleton, who died between 1356 and 1362, the researchers say.

"None of us in the team have ever seen a lead coffin within a stone coffin before," archaeologist Mathew Morris, the Grey Friars site director, said in a statement. "We will now need to work out how to open it safely, as we don't want to damage the contents when we are opening the lid."

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Richard III, the last king of the House of York, reigned from 1483 until 1485, when he was killed in battle during the War of Roses. He received a quick burial at the Grey Friars monastery in Leicester as his defeater, Henry Tudor, ascended to the throne.

Richard's rise to power was controversial. His two young nephews, who had a claim to the throne, vanished from the Tower of London shortly before Richard became king, leading to rumors that he had them killed. After his death, Richard was demonized by the Tudor dynasty and his reputation as a power-hungry, muderous hunchback was cemented in William Shakespeare's play "Richard III." Meanwhile, Grey Friars was destroyed in the 16th century during the Protestant Reformation, and its ruins became somewhat lost to history.

Setting out to find the lost king, archaeologists started digging beneath a parking lot in Leicester last summer where they believed they would find Grey Friars. They soon uncovered the remains of the monastery and a battle-ravaged skeleton that was later confirmed through a DNA analysis to be that of Richard III.

In an effort to learn more about the church where Richard was buried — as well as the other people buried alongside him — a fresh dig at the site began in early July.

A King Richard III visitor center is being built at the site and arrangements are being made to reinter the king's bones. The Cathedral of Leicester recently unveiled its $1.5 million (£1 million) plan to rebury the monarch in a new raised tomb inside the church, with a week of celebrations leading up to the reinterment.

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