The Agriculture Department defended its decision to wait 18 days before seeking the recall of millions of pounds of ground beef after initial tests showed E. coli contamination, saying it was following standard policy to rule out other factors.

In a telephone interview Thursday with The Associated Press, department officials acknowledged that they knew as early as Sept. 7 that frozen hamburger patties could be contaminated after a federal inspector confirmed that preliminary tests indicated the E coli bacteria strain O157:H7.

The department said it was following its common practice of confirming the original results. However, because of the delay in this case, Agriculture officials will now re-examine the policies to determine whether quicker notice is necessary.

"We are concerned about that delay and we recognize that we can do better," said spokeswoman Terri Teuber. "One of the things we're looking at for future recalls is to determine whether the science is strong enough in some cases that we should authoritatively move forward sooner."

The department's response comes after the AP obtained an Agriculture Department e-mail showing the department knew on Sept. 7 about possible contamination but waited 18 days before concluding Topps Meat Co. should issue a recall.

The recall that began Sept. 25 was soon expanded to comprise 21.7 million pounds of hamburger produced by Elizabeth-based Topps, making it the second-largest beef recall in U.S. history.

A Florida teen was hospitalized with kidney failure in August and the meat her family bought was tested by the USDA.

The e-mail — from federal inspector Kis Robertson, an employee of the USDA's Food Safety Inspection Service — was provided by the teen's family lawyer, Scott P. Schlesinger, on Thursday. The e-mail was reported Wednesday by The Chicago Tribune.

"They should have recalled immediately. That's not even a maybe," Schlesinger said.

Asked about the delay, Topps spokeswoman Michele Williams referred questions to the USDA.

"We've been fully cooperating with their investigation, and certainly taking their recommendations," she said.

Teuber told the AP that the Sept. 7 preliminary results yielded 13 negative samples of the questionable ground beef and just one positive result.

Subsequently, USDA scientists conducted a more sophisticated test, known as the Pulsed Field Gel Electrophoresis, that took an additional seven days. After those results became available on Sept. 14, the department then investigated to determine whether the suspected meat could have been contaminated in the consumer's home.

In general, home contamination is rare and the Agriculture Department is now reviewing whether to bypass that test in the future, Teuber said.

"We have real concerns about the 11-day lapse from the 14th to 25th," she said.

As of noon Wednesday, 29 people in eight states had E. coli infections matching the strain found in the Topps patties, the Centers for Disease Control and Prevention reported. None have died.

The first illness began July 5, and the last began on Sept. 11, the CDC said.

The U.S. Department of Agriculture is investigating in an attempt to find the source of the E. coli strain, which stems from the intestines of healthy cattle.

The CDC reported these states and number of cases: Connecticut (2), Florida (1), Indiana (1), Maine (1), New Jersey (6), New York (9), Ohio (1), and Pennsylvania (8).

Privately held Topps, which is believed to be the leading U.S. maker of frozen hamburger patties, said it sells its products to supermarkets and institutions such as schools, hospitals, restaurants and hotels.

Topps gets beef parts from slaughterhouses, grinds them, forms the meat into patties and freezes them.

The recall represents all Topps hamburger products with either a "sell by date" or a "best if used by date" between Sept. 25 this year and Sept. 25, 2008. All recalled products also have a USDA establishment number of EST 9748, which is on the back panel of the package or in the USDA legend. A full list of the recalled products is available at http://www.toppsmeat.com.

The company has said it believes much of the recalled meat has already been eaten. Thorough cooking, to at least 160 degrees, can destroy the bacteria.