DENVER -- Ten Chase banks in Colorado and Oklahoma received threatening letters Monday, some containing white powder.

The FBI in both states said initial tests indicated the powder wasn't dangerous but more testing was planned.

"We're treating each instance as a very serious matter, and the substance is going to the state lab for further testing," said FBI spokeswoman Kathy Wright in Denver.

Six of the seven metro-area Chase Banks that received the letters are:

Denver -- 3300 Colorado Blvd.

Denver -- 1760 16th St.

Lakewood -- 1400 W. Colfax

Arvada -- 8015 Kipling St.

Arapahoe County -- 16746 E. Smoky Hill Road

Westminster -- 94th and Sheridan

Most of the banks were closed Monday afternoon as police and hazmat crews were called in to investigate and decontaminate.

The Chase Bank in Arapahoe County was inside an Albertson's grocery store. After a suspicious package was found at that bank at about 2 p.m., the entire grocery store was evacuated, police said. The grocery store reopened two hours later, but the bank remained closed.

In Oklahoma, two Chase Bank branches in Oklahoma City and one in Norman, Okla., also received threatening letters with white powder, the FBI said.

Gary Johnson, an FBI spokesman in Oklahoma, said the letters received there indicated the threat was "based on past actions of the bank." Wright said she couldn't discuss the contents of the Colorado letters.

Wright said investigators believe the incidents are related. The United States Postal Inspector has also been brought in to investigate the case.

Similar letters destined for other Chase branches in Oklahoma were intercepted before they were opened, Johnson said.

In Norman, at least 10 employees of one bank were decontaminated after an employee opened one of the threatening letters.

Mary Jane Rogers, a spokeswoman for JP Morgan Chase & Co. in Phoenix, said bank officials are cooperating with the FBI and local police in both states.

The bank released the following statement: "Several branches in Colorado and Oklahoma received threatening letters, some with suspicious powder. The branches are in Metro Denver, Oklahoma City and Norman, Oklahoma areas. There are no reported injuries but some employees did request to be examined by a doctor. We take these threats seriously and are working closely with law enforcement."